I think the Valve Wizard site is an excellent place to learn about tube amps.
http://www.freewebs.com/valvewizard/
That and studying Fender amps .. from the beginnings - some very fine and understandable designs through time.
After that - a good parts supplier catalog - like Tube Depot or somewhere.
Then after a while - start buying parts!
And if you are especially new to tube circuits - read up on safety and handling procedures for high voltages
ie. learn to keep own self safe, learn to keep the build safe
After you get past the initial stages, the book "Designing Tube Preamps for Bass or Guitar"
It's the definitive guide for the next stage of understanding preamps.
http://www.freewebs.com/valvewizard2/Book1.html
For a general all round guide to construction and operating principles, Morgan Jones i think ;D
"Valve Amplifiers" and "Building Valve Amplifiers"
If you need more basic instruction, there might be a book or two before all that.
The main thing is to build some simple ones to begin and get more elaborate as you go.
Practice really does improve the results.
Probably the best guides for lower power instrument amps is AX84 site
http://www.ax84.com/
Thats pretty much it. Hope you have some spare funds if you want to become a tube enthusiast
And space.
Cheers
Good luck