For fundamentals of electricity, electronics, and tubes, I have really gotten a lot out of the NEETS (Navy Electricity and Electronics Training Series) modules, available here (
https://www.hnsa.org/manuals-documents/2575-2/) and other locations on the web.
For understanding amplifier operation, a book I found helpful is "Design of an Integrated Push-Pull Tube Amplifier Made Easy" by Giuseppe Amato. There is a companion (or maybe twin) website:
https://www.vtadiy.com/book/ .
There are myriad books, websites, and YouTube channels; so many it's hard to choose. I have gotten a lot out of the Blueglow Electronics YouTube channel - there are many, many videos there that cover various aspects of tube amplifiers as well as new amp builds and repairs or rebuilds of vintage amps.