Servo Theory

Control System Design Guide by George Ellis, 1991, TJ 216 E39 1991
A very nice, readable, and practical book.
Observers in Control System by George Ellis, 200, QA 402.3 E45 2002
Specialized, uses his unique software, very readable.
Stochastic Processes and Filtering Theory, Jazwinski, 1970, QA 276.8 J38
An "egghead" book, all math, no practical information, recommended as mathematical background for Kalman Filter studies.
Linear Estimation and Stochastic Control, Davis, 1977, QA 402.3 D38
Another "egghead" book, but at leas thinner than the above,
Introduction to Random Signals and Applied Kalman Filtering Brown and Hwang, 1992 (2nd edition).
Now here is a nice book with a clean organized mathematical first half.