University taught me most of what I needed to know about the theory of electronics, but not a jot about valves. They had been dropped from the curriculum in the 1970’s in favour of these new integrated circuit things!
Having acquired a Quad Valve system in need of attention I had to learn about Valves.
This was the early 1980’s. No web then. Paper books were the only way to go. Fortunately my then Girlfriends father was an ex-Navy man. He kindly gave me a set of “Common Core Basic Electronics” books. I used these to fill the gaps in my knowledge.
The Common Core books are intended to train Navy technicians. I suppose an early version of the modern advert on TV “If you can fix a skateboard … then you can fix a Samsung Radar”. The emphasis is on understanding and repair rather than design. They acknowledge this themselves in the introduction.
“.. If you are one of those chaps who can absorb and retain everything in these manuals at a single reading, you have no business reading them anyway. A whopping great text book with a B.Sc at the end of it , would in that case be far more your mark”
Unfortunately these books left me as part of the package when I sold the Quad Amps. Recently I have bought replacements for parts 1-3 and will probably get the rest in due course. They are very good.
Another, more concise alternative is “Electronics” by Roland Worcester.
This is full colour illustrated book which straddles Valve and early Transistor technology. Great as an introduction, but maybe not detailed enough if you are getting your hands dirty.
These days of course google is your friend, but for a really good all round electronics text book you can’t go far wrong with “The Art of Electronics” by Paul Horowitz and Winfield Hill. I used this text book back in the early ’80s. It doesn’t cover valves though. My friends son, who is currently studying electronics at Cambridge, also has a copy. The detail may change in 35 years, but “Ye cannee change the laws of physics”. It certainly helped me to my B.Sc, and is currently doing the same for him.