It is quite funny that with the digital/information age so much good information is missing or even harder to find.
Taking a look into the entire audio field there are only a hand full of good techs that have a better than average understanding of what really goes on.
One of the biggest problems epically when it comes to tube audio is that so many people are out there that have a half backed knowledge or sometimes are wrong to start with, but where can "you" get the information that is needed for a good understanding or beginning?
There are only a few books out there that I think explain it right from the start and in a way that personal opinions of the author are not reflected. The pure knowledge of how and why it works is there.
To get started here is the book that any repair tech, do it yourselfer and the interested analog audio fan (analog or digital) should own or at least have read.
Like with most of the audio stuff that is great, it was written in german and only a few English copies were published/are available. I found two over the years in the UK ebay.
The title is
"Rundfunkroehren, Eigenschaften und Anwendungen"
by "L. Ratheiser"
published in several editions from 1937 to 1940, edition 3 and up is recommended.
The book displays all German (European) tubes of the time, with hand down info how a tube is build, how to roughly calculate everything from a diode to a transformer and has all the basis to think about DIY projects.
Aside references to other great books of the time, it has all graphs and diagrams that even a non technician can understand.
It is still easy to find (in german) for about $30 to 50, as already said the English version is quite rare....
It also puts the audio theoretical side into the right perspective, do I really need to get up to 100ooo Hz? with frequency charts of all instruments as well as distortion examples etc.
Plus a chapter that is dedicated to repair work on radio and audio equipment of the time, still valid for analog audio today!
This books was issued with help of the historic Telefunken company, so you find also lots of really cool Telefunken tube pictures!!!!!