WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
The RCA manuals. One favourite: Radiotron Designer's Handbook' an Australian book I think.
Edit: oh, not sure these are good for repair. But they are great books to have by the bed and in the single-sitting room.