Here are my suggestions, in order of easy to digest:
Tube Audio:
Beginner's Guide to Tube Audio Design, Bruce Rozenblit
Principles of Power (A Practical Guide to Tube Power Amplifier Design), Kevin O'Connor
Valve Amplifiers, Morgan Jones
The Tube Preamp Cookbook, Allen Wright
Radiotron Designer's Handbook, F. Langford-Smith
Basic Electronics:
Beginners Guide to Electronics, Martin Pickering
Also:
Glass Audio reprinted numerous articles (Audio Classroom series) by Norman Crowhurst on tube audio design which may still be found online
Back Issues of Vacuum Tube Valley (out of print) which can found as pdfs online, I think
Back Issues of Sound Practices (out of print), don't know if these are available online