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.