Here is the motherload of tube amp related books:
Peter Millett's Tech Books Online
There is enough there to keep you busy for a long time.
Here is another good book to start with:
Jack Darr's Guitar Amplifier Handbook
This is a good, basic intro to tube amps. It was written in the 60s and later expanded in the early 70s so there is no discussion of FX loops, channel switching, or distortion circuits, but it covers the basics of how tubes and amps work, and how to fix them. It is also available in print, and it is well worth it to have a hard copy as the above .pdf files only are about half of the book. The other half is a bunch of 60s and 70s era tube amp schematics, which make the book well worth the price.