Howard,I would recommend 3 books to you. All available on the net.
1) book by Morgan Jones (forget title)
2) Tube amp cook book by Allen Wright (Vacuum state.com)
3) Audio Reality by Bruce Rosenblitz (Transedent OTL fame)
All 3 of the above are highly respected, have different points of veiw, discuss why they choose the parts they did.
Have fun
Joe