Back in the days when I had my shop in SoCal, I did a fair amount of vintage guitar tube amplifier repair. I found a couple of must have books on the topic. Foremost amongst these books is "A Desktop Reference of Hip Vintage Guitar Amplifiers" by Gerald Weber. Comprehensive in it's scope and valuable as a reference it has pages of vintage schematics and recipes for popular mods for popular vintage amps. Even if you never install one of the mods it is nice to know what they are and what they do since the chances of seeing a modded amp are relatively high if you are repairing them on a regular basis.

"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.