I learned how to fix tube-type radios when I was in high school back in the late 1960s. I used a 1932 book called "Ghirardi's Radio Physics Course" by Alfred P Ghirardi that I found in my school library.
I'm told the best new text available today is "Fixing Up Nice Old Radios" by Ed Romney. Romney is a retired radio instructor. The book retails for $35.
Radio repair is really quite easy once you master the basic principles and use some common sense. You'll also need a volt-ohm-meter, a tube tester, and a supply of resistors and capacitors. I built my original parts stock by stripping old TVs I found at the dump.