Resources

I really don't understand how you could ever land on this page without knowing about these resources first, but just in case you did:

Uncle Doug's YouTube channel

After watching a few of his videos, you'll be able to tell that he's a very passionate person who loves what he does. He's also very good at explaining things in a way that's easy to understand.
So easy, that I got myself convinced that I could build my own tube amp.

The Valve Wizard (Merlin Blencowe)

I hope not to seem too much like a fanboy, but Merlin wrote what I personally think is the bible of tube amp design.
There are a lot of resources out there, but I think this is the best one to start with.
On his site you can find a lot of "starter" information which is also contained in the books, but they are actually the real deal, I'm talking especially about the "Designing Tube Preamps for Guitar and Bass" and "Designing Power Supplies for Tube Amplifiers" books.
What makes them so interesting is that they both convey the necessary theory, but it's also full of "common knowledge" that you can only get from experience, alongside a lot of schematics ready to use.

Psionic Audio

A lot of great videos that show how a professional works.

Blueglow Electronics

Not strictly a guitar amp channels (more toward the Hi-Fi side indeed), but he's got a lot of great videos explaining topics about theory and practical tasks that one may encounter while building an amp.

Skunkie Designs Electronics

An awsome channel about custom amp builds with a great hands-on approach.

Jolo Carrera

This guy is one of the people that have built the same amp I'm building, and I've taken a lot of inspiration from his videos.