Following is a list of "must have" books for the serious tube amplifier designer. Some of them are long out of print, and can only be obtained from used book sellers, although they are occasionally seen for sale on the internet. The ones marked with an asterisk are the absolute, "can't do without" books, in my opinion. The other books on the list are also well worth finding. There are other books out there which may also be good, but I can't comment on them if I haven't read them.
A Desktop Reference of Hip Vintage Guitar Amps - Gerald Weber. Published by Kendrick Books, 1994. ISBN 0-9641060-0-0. Lots of good schematics, primarily Fender-oriented, but some other amps as well. In addition, it contains a lot of mod information for Fender amps, if you are into that kind of thing.
* Audio Cyclopedia, 2nd Edition - Howard Tremaine. Published by Howard W. Sams & Co., Inc. The Bobbs-Merrill Co. Inc., 1974. International Standard Book Number 0-672-20675-7, Library of Congress Catalog Card Number: 77-82885. This huge volume is arguably the best book ever published on tube amplifier circuits and other electronic circuitry, from filters to test equipment and everything in between. It is presented in a question-and-answer format, and contains concise, easy-to-understand explanations of almost every tube circuit known to man (some of the newer patents on amp circuitry bear a suspicious resemblance to circuits in this book). Don't buy the later, solid-state edition, the 2nd edition is the one you want. Good luck finding a copy, as it is long out of print.
Audio Transformer Design Manual - Robert G. Wolpert. 1989. This book explains audio transformer design in great detail. There is also a companion book on power transformer design. These books were out of print for several years, after Mr. Wolpert's passing, but his daughter has made them available again through this website link: http://www.rgwdesign.com
Build Your Own Audio Valve Amplifiers - Rainer zur Linde. Published by Elektor Electronics, 1995. ISBN 0-905705-39-4. High-level technical information on vacuum tube circuit design.
Dave Funk's Tube Amp Workbook - Dave Funk. Published by Thunderfunk Labs, Inc. 1996. ISBN 0-9650841-0-8. A good beginner's text on tube amp design.
*Designing Tube Preamps for Guitar and Bass - Merlin Blencowe. Published by Merlin Blencowe, 2009. ISBN 978-0-9561545-0-7. This is an excellent book, packed with concise, factual information on tube (valve) preamp designs, complete with all mathematical formulas for design. This book is destined to be a classic, and sort of reminds me of a Radiotron Designer's Handbook for guitar. Highly recommended. If you can only afford one book, make it this one.
*Designing Power Supplies for Tube Amplifiers - Merlin Blencowe. Published by Merlin Blencowe, 2010. ISBN 978-0956154514. I haven't seen this book yet, but based on the quality of the first Blencowe book, I reocmmend it sight unseen!
Fender Amps - The First Fifty Years - John Teagle & John Sprung, published by Hal Leonard Corporation, 1995. ISBN 0-7935-3733-9 (soft cover), ISBN 0-7935-4408-4 (hard cover limited edition). The title says it all. Very nicely done history of Fender amps.
Electric Guitar Amplifier Handbook, 2nd edition - Jack Darr. Published by Howard W. Sams & Co., Inc. Indianapolis, Indiana, 1968. Library of Congress Catalog Card Number: 68-59059. There is also a much better and larger third edition: ISBN 0-672-20848-2, Library of Congress Catalog Card number: 78-157801. This is an excellent little book with very good, easy-to-understand theory and troubleshooting tips.
Fundamentals Of Vacuum Tubes, 2nd Edition - Austin V. Eastman. Published by McGraw-Hill Book Company, Inc., 1941. A textbook on vacuum tubes, highly technical.
*Guitar Amplifier Preamps - Richard Kuehnel. Published by Pentode Press, 2009. ISBN 9780976982227. This is another excellent book. It deals with guitar amplifier preamp designs from an engineering standpoint, and contains derivations of formulas for all phases of preamp design. If you want to know the math behind the circuit design, this book is it. It is not just math, however, it also is filled with practical information. Highly recommended.
*Guitar Amplifier Power Amps - Richard Kuehnel. Published by Pentode Press, 2008. ISBN 9780976982241. This is another excellent book, a companion to the preamp book listed above. It deals with guitar amplifier power amp designs from an engineering standpoint, and contains derivations of formulas for all phases of power amplifier design. Highly recommended.
Noise Reduction Techniques In Electronic Systems - Henry W. Ott. Published by John Wiley & Sons, 1976. ISBN 0-471-65726-3. A textbook on noise reduction, highly technical.
Principles of Electronics - M. R. Gavin and J. E. Houldin. Published by D. Van Nostrand Company, Inc., 1959. A very nice little book written in easy-to-understand, yet technical, language.
Principles of Electron Tubes - Herbert J. Reich. Reprint published by Audio Amateur Press, 1995. ISBN 1-882580-07-9. A classic text, but hard reading. For advanced folks only.
Principles of Power - Kevin O'Connor. Published by Power Press Publishing, London, Canada. ISBN 0-9698-6081-1. A good tutorial on tube power amp design.
Pulse Electronics - Raphael Littauer. Published by McGraw-Hill Book Company, 1965. Library of Congress Catalog Card Number 64-22195. Mainly geared towards pulse generation, but has some good tube amplifier information.
Radio Designer's Handbook (aka Radiotron Designer's Handbook, 3rd Edition) - F. Langford-Smith. Published by the Wireless Press for Amalgamated Wireless Valve Company Pty. Ltd., 1953. Reproduced and distributed by RCA in America. This is a little blue book, much smaller than the 4th edition (only 352 pages), but it contains quite good information, and is well worth getting if you don't have the 4th edition, or even if you do have it. This edition is long out of print and is seldom seen for sale.
* Radiotron Designer's Handbook, 4th Edition - F. Langford-Smith. Published by the Wireless Press for Amalgamated Wireless Valve Company Pty. Ltd., 1953. Reproduced and distributed by RCA in America. This book is the "granddaddy" of them all. If you understand everything in this book, you will be a true guru. The original book is long out of print, and will cost $50-$150 for a good used copy. However, Antique Electronics has issued a nice hardback reprint for $69.95. They can be found on the web at http://www.tubesandmore.com/. It has also been reissued by Butterworth-Heinemann (1999) and is available on Amazon.com - ISBN 0750636351, for $79.99.
*The History Of Marshall - Michael Doyle. Published by Hal Leonard Publishing Corporation, 1993. ISBN 0-7935-2509-8. If you are a Marshall fanatic like me, this book is on the top of the list. It contains a wealth of information, including pictures, schematics, and a full section on Celestion speakers. Highly recommended.
* The Tube Amp Book, 4th Edition - Aspen Pittman. Published by Groove Tubes Audio, 1993. There is also a newer 4.1th edition. Get this book for the extensive collection of schematics and cool amp pictures, ignore the sales pitches and especially the tech info, as much of it is incorrect. This is your best schematic reference book.
The Ultimate Tone - Kevin O'Connor. Published by Power Press Publishing, London, Canada. ISBN 0-9698-6080-3. This book is a good source of detailed circuit design and tube amplifier modification information. There are occasional technical errors, but overall, it is good. The companion book, Principles of Power, is also good. There are several other volumes in the series, but most deal with esoteric information of little use to the average experimenter, and they are quite pricey, upwards of $100 or so.
The Vox Story - David Petersen & Dick Denney. Published by The Bold Strummer, Ltd., 1993. ISBN 0-933224-70-2. A book on all things Vox, very well done.
* Valve Amplifiers - Morgan Jones. Published by Newnes, Butterworth-Heinemann, Ltd. 1995. ISBN 0-7506-2337-3. An excellent tube theory book, highly technical, but relatively easy to understand. It contains a lot of the math behind tube circuits, but in easy-to-understand form. Highly recommended. This book has been continuously in print since 1995, and is now in it's third edition. The ISBN number for the third edition is: ISBN 07506 56948. There is also an upcoming book entitled: "Building Valve Amplifiers" (ISBN 07506 56956) that is due out any day now.