Tube Testers and Classic Electronic Test Gear by Alan Douglas. 2000
Getting the Most Out of Vacuum Tubes by Robert B. Tomer. 1960
Inside the Vacuum Tube by John F. Rider. 1947
Tubebooks.org - has most editions of the RCA manuals, including the huge HB-3 Handbook, as well as manuals from Sylvania, Tung-Sol, and others. There are also a number of books that may be useful, such as the third and fourth editions of the Radiotron Designer's Handbook. All are free.
Vacuum Tube Era - has a collection of scans of a variety of books and pamphlets, including the 1945 Sylvania tube book, various substitution guides, and all sorts of other things. All are free.
Antique Radios - is both a front-end and a huge forum, both of which are full of information. Check out the Archive in particular.
and there are more resources of all kinds on my Links page.