This book, now long out of print, is still considered a solid reference on the care and feeding of guitar amps. This is offered as PDF downloads and is split into chapters:
SECTION I – How Guitar Amplifiers Work
Chapter 1 – Amplifying The Signal
Chapter 2 – Special Signal Circuits
Chapter 3 – The Power Supply
SECTION II – Service Procedures and Techniques
Chapter 4 – The Amplifier Signal Circuits
Chapter 5 – The Power Supply
Chapter 6 – Output Stage, Transformers and Speakers
Chapter 7 – Cables and Pickups
Chapter 8 – Customer Complaints