"The Tube Amp Book" is the classic book on tube amp repair by the founder of Groove Tubes, Aspen Pittman. This has been the quintessential reference manual for guitar amp shops since the first edition in 1986. The book is now available in a "deluxe revised version" from Amazon at a substantial discount. I have not seen this edition but I own the 4th edition of the original and it has been well used.