Hi there,
I am also a beginner in theory and very new to building my own amps, I had my eye on the Aspen Pittman Tube Amp Book for a long time and always wanted it and finally found a cheap copy
But it doesn't have much if any theory in it, for me as a beginner especially, 3 pages of biasing tips, a section from Jack Darrs book [posted above] and a small section on mods for Fender/Marshall and Ampeg,
But it is fantastic for schematics [ I can only just read those properly now] and other reference, tube values etc, it can be expensive though for those older copies, and I'm not sure what the new edition is like, it may have more in there now
The Kendrick/ Gerald Weber DVD recommended above taught me in a few months, more than I learned in 5/10 years! Its just the way he explains power flow, signal flow, AC/DC current and the basics of transformers and tube theory worked for me and my mind [ its that Texas drawl just pulls me in...]
So for me with little theory of the amps workings or power section/safety the best books for me were:-
-Merlin Blencowe "Designing Valve Preamps for Guitar and Bass"
-Gerald Weber "Tube Guitar Amplifier Essentials"
and I'll throw in Dan Torres "Inside Tube Amps" too if you see it for a good price!
My best videos are:-
Gerald Weber "Understanding Vacuum Tube Guitar Amplifiers" DVD
Gerald Weber "Advanced Tube Guitar Amplifier Servicing and Overhaul" [Has a good slow explanation to help me with transformers, finding a mystery transformers wirings and taps, and working out an output transformers ratios by inputting voltage etc]
Uncle Doug on Youtube, his theory videos on power and single ended amps helped me,
I am getting started in the world of tube amps. I am working through getting an old Gretsch up and running,
Also thumbs up for safety first, I am recommending these, you know what they say about curious cats, high voltage etc
As I have never found a good demo of the spark from discharging caps, out of all the videos I have watched for years, they were always still pictures or diagrams to show where to discharge with a screwdriver, or what kind of lead or probe to make, and not many recommend putting a resistor on there and showing the actual discharging with voltage etc,
Please be careful out there, and read as much safety information on mains power and filter caps as possible first, even those times you put your fingers inside to pull the chassis out of the box on that ol' Gretsch
I have been lucky so far, but saw my grandpa fly across a room as a kid.... He was ok after, but kinda put me off fiddling with things for a while!