Paleoelectronics.com is now hosting a mirror of the famous Radiontron Designer’s Handbook, 4th. ed.
This is classic of valve design although it’s a bit weird (it teaches you electronics after it biases tubes!). Still it’s great to have around. I recommend the hardcopy (always on ebay). It’s only 1400 pages, just take it everywhere!
I have had the 4th Edition for more than 40 years! And I still refer to it sometimes. The basics have not changed, Ohm’s law still works, coupled tuned circuits still behave the same way and transformer theory still holds. And for some, valve amplifiers are in vogue again, so this is a good reference.