Introduction

There are many introductory books and courses on locksmithing available.  It’s important to note that any trade that can be learned by simply reading a book is hardly a trade worth getting into, and locksmithing is no exception.  There is no substitute for extensive hands-on experience.  At best, these books and courses let you know if locksmithing is something you are really interested in. Additionally, they can impart some of the lingo and basic techniques of the trade.  Reading through them would be the first of many steps towards becoming a proficient locksmith.
As a final note, these books and courses can be ordered by bookstores, are at some libraries, and can usually be found used on eBay, Amazon, Barnes and Noble, and other book dealers. While I do mention certain businesses in specific context it does not mean these books are limited to that specific business.
With that said, here are introductory locksmithing books and courses ranked in order of desirability, based strictly on my opinion:

First Place:  

First place goes to buying a used Locksmithing Institute or Foley Belsaw course on eBay.  Sometimes they do not go for much money.  They, like every other book, do not cover everything, and do not cover new developments, but they tend to cover the basics very well and they are well illustrated.  If buying a really old copy, note that filing down lock pins should not be done these days given the pinning kits now available.  Buying one of those old binders, collecting a box of old locks to practice on, and buying some basic locksmith tools would be a good start. 

Second Place: 

Locksmithing From Apprentice to Master, Joseph Rathjen, Copyright 1994, ISBN 0-07-051645-6, 309 pages, 7″ X 9″.
Table of Contents:
1. Locksmithing Intro / Getting Started
2. Tools
3. Jobs / Certifications / Law / Setting up Shop
4. Types of Locks
5. Lock Functions
6. High Security Locks
7. Door Closers
8. Keys, Spacings and Depths
9. Master Keying
10. Interchangeable Cores
11. Automotive Locksmithing
12. Alarms
This book has its weak points and strong points. The weaknesses are:
1. The title includes “from apprentice to master”. The “master” should have been left off. I think the book has some good parts, but master locksmithing it is not.
2. It does not go over picking, impressioning, or bypassing–Rathjen did not believe in letting that information out.  Fair enough, but an aspiring locksmith would need to learn about these subjects somewhere else.
3. The automotive section is 20 years out of date, and does not go over opening cars–a traditional source of revenue for new locksmiths.
4. It has a door closer section, but it is very light. It does not go over door closer sizes, does not mention ADA (which had become law a few years before this book was published), etc. Maybe better than nothing but not really enough for one to offer one’s services for door closer installation and repair.
5. It has a section on going into business for oneself but seems to be more oriented towards setting up a storefront than establishing a mobile service. Being published 20 years ago it cannot help being out of date on advertising and such.
The strengths are:
1. It does not have a lot of filler material and the writing is direct and to the point.  A lot of information per page. 
2. It has a twenty page section on master keying, and I think it would be enough to actually get someone started with the rotating constant progression method.
3. It has a 17 page section on interchangeable core locks. It goes in detail on how to work with Best SFIC, and briefly touches on Schlage and Medecco.
4. It has an almost 50 page section on alarms. It goes over hardwired systems. From designing the system, setting up the panel, running wires, installing sensors, etc.  It is thorough for when it was written and probably still good background information for someone interested in them.  
It is long out of print, but can be picked up cheap from Amazon and other used book sellers.  At the current used prices, this book gives the most bang for the buck.   

Third Place (tie): 

The Complete Book of Locks and Locksmithing, 7th Edition, by Bill Phillips, copyright 2017, ISBN 978-1-25-983468-4, 648 pages, 7 1/4″ X 9″
Table of Contents:
1. Lock history
2. Tools
3. Types of locks and keys
4. Warded locks
5. Lever tumbler locks
6. Disc tumbler locks
7. Pin tumbler locks
8. High security locks
9. Masterkeying
10. Smart Locks
11. Buying and selling safes
12. Keyed padlocks
13. Home and business services
14. Lock picking and impressioning
15. Automotive
16. Opening locked cars
17. Forced entry
18. Combination locks
19. Electrical access control
20. Working as a locksmith
21. Key duplication machines
This book is the first locksmithing book that a lot of beginners read. It is fairly comprehensive. It is published by McGraw-Hill, and is marketed to bookstores, libraries, etc. First, some background on the book. The first couple of editions were written by C.A. Roper. Then Roper and Phillips co-authored. Then later on, Phillips was listed as the sole author. Do not be surprised if an early edition lists Roper as the author. 
The strength of the book is that it does a fairly good job of going over the basic basics.  Clear writing and good illustrations.  The weakness of the book is that parts of it are very shallow, other parts are dated, and it is bloated with material cut-and-paste from manufacturer’s literature.  Literature that anyone can download for free off the internet. As an example, the section on automotive is badly dated.  It goes over AMC cars and the latest technology it discusses is GM VATS.  Nothing about transponder technology and nothing about modern opening tools. 
In the page after page of cut-and-paste filler, it devotes 22 pages to the installation instructions of the Schlage model G.  That lockset went out of production in 1981.  Meanwhile, it has nothing about, for example, the Kwikset Smartkey locks.  In electronic locks, it goes over Dialocs and Memorilocks, but does not mention the Alarm Lock Trilogy. 
It does not go over door closers and does not have much by way of life safety codes, the ADA, etc.  The chapter on working as a locksmith makes no mention of being a mobile locksmith and advertising is mainly concerned about hanging a good sign in front of the shop. 
The book really does not go into detail about cutting keys by code.  While it goes over the fairly uncommon Framon DBM-1 flat key duplicator, it does not mention the Framon #2 or the HPC 1200 which are very common code machines for locksmiths, new and old. 
There is no meaningful discussion of interchangeable core locks. It gives a picture of one, but does not really go into enough detail to actually do anything.
Where the book does have value is in a fairly comprehensive treatment of the basics: types of locks, how they work, how to rekey them, etc. Basically what was in the 2nd edition from 1983. Bottom line, if you want the basics in one book, then get The Complete Book of Locks and Locksmithing, but get the cheapest edition you can find.  Early editions go for almost nothing used and they have the basics, but not the filler. 

Third Place (tie): 

Locksmithing, Bill Phillips, Copyright 2010, ISBN 978-0-07-162275-2, 412 pages 7″ X 9″.
Table of Contents:
1. Business of locksmithing
2. Types of locks and keys
3. Key blanks
4. Warded, lever tumbler, disc tumbler, and side bar wafer locks
5. Pin tumblers
6. Tools
7. Key-in-knob, deadbolt, and cylinder mortise locks
8. High security locks (CoreKey, DOM, Kaba Gemini, Medeco, and Schlage Primus)
9. Simplex Locks
10. Picking, impressioning, and bypassing
11. Masterkeying
12. Safe basics
13. Drilling safes
14. Key bumping
15. Key code machines (KD80, Exacta, Borkey 989, and Framon DC-300)
16. Automotive Locksmithing
17. CCTV
18. Access Control, Alarms, and Systems Integration
19. Working as a Locksmith
This book is basically The Complete Book of Locks and Locksmithing but with a couple of hundred pages of filler removed.  It may be easier for someone new to the trade to read through—it seems to be a little better organized in that regard.  The sections on automotive and key machines are no better than the other book.  CCTV and Access control chapters are high level, not much practical information in them. 

Fifth Place: 

Shankle and Shankle Comprehensive Manual of Locksmthing, R.H. and M.D Shankle, Copyright 1994, ISBN 0-9640733-0-7, 584 pages, 7″ X 10″.
Table of Contents:
1. History of Locks
2. Pin Tumbler Locks and Keys
3. Basic Tools
4. Key Machines — Basically the handbooks for the Ilco 017, the MK2 Exacta Punch, and the Framon DBM-1 Flat Key Machine.
5. Types of Locks and Keys
6. Services — Goes over work that Locksmiths perform, then goes into detail on rekeying common door locks. Almost 100 pages in this chapter.
7. High Security Locks and Keys — Medeco (40 pages), Corbin Emhart, Schlage Primus, Assa Twin, and DOM.
8. Disc Wafer Locks and Keys
9. Master Keying — There is enough to this chapter to enable one to actually create a simple master key system.
10. Double Bitted Locks and Keys — (Doubled sided wafer locks)
11. Warded Locks and Keys
12. Lever Locks and Keys
13. Automotive Locks and Keys — With the book being more than 20 years old it is out of date. VATS is the latest technology in the chapter. No transponder keys.
14. Vending Machine Locks and Keys — Tubular Locks
15. Combination Locks — For an intro book a fairly thorough section. Nothing on safe opening, but lots on combination changing and troubleshooting from an S&G handbook.
16. Pad Locks — How to re-key pad locks. Includes Abloy Disklocks.
17. Lock Decoding — A couple of paragraphs on reading wafer locks, and a few pages on the Lee Decoder Kit for lever locks.
18. Key Blank Reference. OK, but anyone serious about the subject would have the latest catalog from Ilco or Jet.
19. Lock Picking — Half a dozen pages, very basic. Nothing on security pins.
20. Law Related to Locksmithing — One page.
21. Starting a Locksmith Business — May have been good advice twenty years ago. Only marketing mentioned is the Yellow Pages.
22. Electronics — Access control, cameras, and an interesting section on fence intrusion detection systems.
This 1994 book might be seen as an alternate to Phillip’s book on locks and locksmithing. Compared to that book, it tends to be a little more advanced and practical in places, but may not be as a good a book for a complete beginner. It was self-published and is out of print, but used copies show up on Amazon and eBay.
It was probably written as both an introduction to locksmithing and as a handy all-in-one reference source for common locks. Where this books suffers is that it is not very well organized and might not be the best choice for a beginner. It is all there but skips around a bit. As a reference, it does have an index so at least one can find things. However, it was written a few years before the Internet became ubiquitous and much of the reference material can be found on-line.
Overall, if a beginner could only have one book, this would not be a bad one to have. But there are better books out there for beginners and as a reference source, it was a good idea in 1994, but much of the reference sections are available on-line these days. 

Sixth Place: 

Master Locksmithing, Bill Phillips, Copyright 2008, ISBN 978-0-07-148751-1, 9″ X 7″, 416 pages.
Table of Contents:
1. Types of locks (refresher)
2. Picking pin tumblers
3. Bumping
4. Impressioning
5. High Security Locks
6. Rekeying Kwiksets
7. Non-locking Door Hardware
8. Electronics
9. Emergency Exit Devices
10. Electric Strikes
11. Alarms
12. Magnetic Locks
13. CCTV
14. Access Control
15. Automotive Locks
16. Masterkeying
17. Safes – Buy and Sell
18. Safes – Drilling and Manipulating
19. Working as a Locksmith
20. Safe and Secure Home
21. Security Consulting
22. Appendixes: Finish Codes, assorted spaces and depths, suppliers, electrical schematics, suppliers, etc.
Phillips has a tendency to cut-and-paste a lot and in this book he goes on a rampage. One of the worst examples is reprinting 40 pages worth of a Kwikset guide on rekeying. First, rekeying the 400 Series Kwikset can be done by a lot of hardware stores—it is not the stuff of master locksmiths.  Second, if one wanted the Kwikset guide to rekeying, it is free to download anyway.
The most ridiculous cut-and-paste was the appendix on electrical schematic symbols—it is  apparently copied from an old amateur radio handbook.  How often do dipole antennas, headphones, telegraph keys, phono-jacks, and pentode vacuum tubes show up in locksmithing?  Pure filler.  
The bumping chapter is more cut and paste: nasty-grams between AOLA and Marc Tobias. One page on how to bump, and the solution to bumping is not to sell bump keys to the public. Nothing about bump resistant locks or how to make them bump resistant.
The impressioning chapter was very light on text, very basic. The high security chapter was about a dozen pages reprinted from ASSA. The electronics chapter was very basic, high school stuff on Ohms law. The emergency exits chapter was 16 pages of a handbook from Alarm Lock.  The electric strike chapter was one of the better chapters, about twenty pages from Adams-Rite.
The alarm systems chapter was very high-level. The magnetic lock chapter was mostly a reprint from Magnalock. CCTV was mostly useless. It does not even mention DVR’s or IP networked cameras. This was published in 2008 and it sort of implies that the only way to record footage is via a VHS recorder on 6 hour tapes.
Access Control was two pages. Automotive locks was very basic. Masterkeying started with a strange excursion into warded and lever locks, then 6 pages on pin tumblers. Not enough material to get anyone started, in my opinion.
Selling safes goes over some safe terminology. Drilling and Manipulating–finally, something exciting for an aspiring locksmith? It has a few high level pages about how safes can be drilled and there is one page on the concept of manipulation, but not enough to really do anything. After reading the Master Locksmithing chapter on drilling and manipulating the reader would be stumped by a SentrySafe from Walmart.  Granted, it is good that a book that would get a person started drilling safes is not sold to the public, but the book gives false hope that the reader will be able to do something useful with opening safes, and it does not.
Overall, it is bloated with manufacturer literature that is freely available and the material otherwise in the book is so high-level that I doubt that anyone could actually put much of it into practice, at least intially.  

Seventh Place: 

Complete Course In Professional Locksmithing by Robert Robinson, Published by Nelson-Hall in 1973. ISBN 0-911012-15-X, 399 pages, 8 1/2″ X 10″.
Table of Contents:
1. Mortise Lock and Panic Exit Device Construction
2. Rim and Cylindrical Locks
3. Key Operated Mechanisms
4. Utility Locks
5. Environmental Servicing
6. Key Duplicating and Code Key Cutting
7. Lock Coding and Masterkeying
8. Opening Doors, Equipment, and Automobiles
9. Lock Engineering Standards
10. Repair Techniques
11. Electric Locks
12. Locksmith and Locksmith Shops
This book is long out of print and is expensive used on Amazon and eBay. The average price is ~$100. Seeing as this book was going for much more used than comparable books new, is there anything to it? Could this be a long lost introductory book that is actually well-written and complete? Is this a hugely useful book or is it a sort of mass-hysteria on Amazon where sometimes out-of-print used books are priced sky high?  I borrowed a copy through interlibrary loan to check it out.
On basic topics, I think the book is a little weak. But it has two bright spots:
1. The first surprise is the first ~50 pages are on mortise locks. Extensive coverage of the topic.  Might be useful for someone repairing an old lock. 
2. The section on lever tumbler locks (under Key Operated Mechanisms) is good, including some things I had not seen before. The book also touches on master keying lever tumbler locks.The lockout chapter is interesting–it would seem that a lot of the material from this book found its way into the Desert Publication Lockout, and Phillips credited Lockout in his chapter on lockouts in his Complete Book of Locks and Locksmithing. So making bypass techniques public knowledge went back to at least 1973 to this book.  
There is some cut-and-pate in the book, for example, the Schlage handbook on wafer locks is in it. But overall, less cut and paste than other introductory texts.
Robinson does devote a few short chapters to lock design. Some engineering principles, corrosion, wear, etc. There is also a chapter on welding, riveting, etc., to rebuild parts. In both cases I’m not sure if there is enough meat there for someone to really make use of it, but there it is. Has some quaint photos of a VW minibus set up as a locksmith van.
But in the end, no, the book is not a lost gem worth the high prices some people are asking. There is a sort of mass-hysteria sometimes on Amazon and eBay with out-of-print books. Just because a book is rare or expensive does not mean it is good, it may just mean that the sellers chose to list it at that price point. The sellers know nothing of locksmithing, they have never read the book, they just know that it is out of print and someone else is listing it for big bucks, so they will list it for about that much too. Personally, I might add it to my locksmithing library if I found a used copy for $10. 

Eighth Place: 

Practical Course in Modern Locksmithing, Whitcomb Crichton, Copyright 1943 – 1971, 222 pages 6″ X 9″, no ISBN.
Table of Contents:
1. Introduction to the Trade – glossary of terms, how doors are handed, etc.
2. Tools and Equipment.
3. Warded Locks – a page on how to fit keys.
4. Lever Locks – several pages on fitting keys and service.
5. Disc Tumbler Locks – briefly goes over disc tumbler, the Schalge wafer tumbler locks, and mentions a short-lived crushable self-keying disc lock.
6. Pin Tumbler Locks – discusses pinning locks, suggests filing down pins (common back then).
7. Masterkeying – really likes Corbin with all their different key ways, so each floor of a building can have a different key way. Does not really go into how to actually masterkey.
8. Service Hints – mentions Best SFIC, cutting keys by code, etc. Mention being the operable word.
9. Safes- discusses how to change combinations, how they operate, etc.
This book shows up used on Amazon every now and then for a few dollars. On the title page it notes that it is “A Benj. Franklin Home-Study Course Complete in One Volume.” That sounds promising. A 40+ year old book is obviously dated, but some elements of locksmithing have not changed since then.
The book is lacking in is anything to do with picking, impressioning, or bypass. It was also strangely lacking in automotive locksmithing, back when a locksmith could just about do anything that needed doing to a car with minimal tools. The page count was 222, but the lower margin is about 1 1/2″ so it is really smaller than it seems.
Overall, there is nothing very useful in the book, not now and probably not when it was published either. What was discussed tended to be at a high level. More like a long pamphlet to be handed out by high-school career counselors. It is of interest possibly as “locksmith lore”, a look back on what the trade used to be. For example, there is a suggested list of hand tools, to take on service calls in something like a doctor’s bag, and the writer reminisces about carrying such a tool kit to service calls via streetcar.