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.

Now this is not a book to learn the fundamentals of tube theory. I would recommend Cy's online tutorial, available to members on this site, or if you could grab a copy of the classic "RCA Receiving Tube Manual" (1975) which is also among my treasured (and well worn) possessions. This book, however, deals with the classic amplifiers and their component parts, the tubes, the speakers, etc. that made them special. This book can give you the knowledge, familiarity, and vocabulary that tells your customers that you know what you are talking about when they deliver their prize amplifier to your care. The book is worth the purchase price if it contained nothing more than the 400+ pages of schematics and the data sheets for the most popular tubes used in guitar amplifiers. But it does contain so much more.

The book is available at Amazon at substantial discount and is a worthy investment for those who are serious about vintage guitar amp repair.