Tomer's Vacuum Tube book
This is THE classic book on how to get the most out of vacuum tubes, how to extend their lives, and information on what causes them to fail prematurely.
Tomer used large amounts of data from industries that used lots of tubes in mission-critical equipment, using the best tubes ever manufactured, including both U.S. and European...which means this book is the ONLY book I know that actually presents scientifically derived data and deductions on how best to use and protect vacuum tubes, instead of the voodoo promulgated by more modern manufacturers who want to cut costs in design.
If you care about tubes, I strongly recommend you buy this book and read it five times. Then design accordingly; the tubes, your clients and your heirs will thank you.
