Inside the vacuul tube - Rider, 1945 - This is a fabulous book on how tubes work and how to design tube circuits. this is NOT a college text; rather this is written more at the level of an advanced technician, with not so much math and lots of diagrams that make visualizing what's going on easy. It even has some 3-D drawings !

Fundamental principles of vibrator power supply design

Basic Theory and Applications of Electron Tubes - US Army, 1952 - This is an excellent, relatively modern text on vacuum tubes.  Very practical, not too much math, and it has some very nice illustrations.  Very specific to tubes, discusses how they work, how they're built, and circuits.

Radiotron Designer's Handbook, 5th edition, 1953 - The later and much more comprehensive version of the RDH3, this is probably the best book out there for the casual vacuum tube circuit designer. Coveted by the audio guys, it also has plenty of info on the design of radio receivers. Covers all the essential of tube electronics design, including the basics, tube theory, load lines, amplifier design, etc.

Theory and Applications of Electron tubes - Reich, 2nd ed., 1941 - 716 pages of vacuum tube theory, this may be THE best book about vacuum tubes. Detailed, complete with plenty of math and drawings. Covers how tubes work and how to design circuits around them, with chapters on class A/AB1 and AB2/B amplifier design, power supplies, and more!

Principles of electron tubes - 1941, Reich - Essentially an abridged version of Reich's "Theory and Applications of Electron Tubes", this is a college-level text on vacuum tubes.  It doesn't have so much math in it to be hard to understand; though it presents plenty of equations, it also shows graphical solutions that the DIY'er could use.  One of the best books I know of to learn enough about how tubes really work to be able to design with them.

Vacuum tube oscillators - 1953, Edson - 488 pages of design details about tube oscillators, including crystal-controlled, variable, relaxation, and other types.

Capacitors, Magnetic Circuits, and Transformers - Matsch, 1964 - A detailed text on capacitors, inductors, and transformers. Great info for those wanting a deep understanding of these passive components. Good theory and practical applications, especially on transformers and inductors.