Guytronix recommends the following 3 books written by Tino Zottola:
Building Vacuum Tube Guitar and Bass Amplifiers – Volume 1
Building Vacuum Tube Guitar and Bass Amplifiers – Volume 2
Building Vacuum Tube Guitar and Bass Amplifiers – Volume 3
New! Only Available at Guytronix.com
Vacuum Tube Guitar and Bass Amplifier Theory by Tino Zottola
Vacuum Tube Guitar and Bass Amplifier Servicing by Tino Zottola