Some of my favorite engineering references (this will be continually updated):

Please let me know if any of the links are broken or if you can find a better version online somewhere.

Digital Signal Processing

The Scientist and Engineer’s Guide to Digital Signal Processing, By Steven W. Smith, Ph.D.

This is a great reference for all things DSP, especially the chapters on Laplace and Fourier transforms. The book is available for free online in multiple places. My favorite is through Analog Devices, http://www.analog.com/en/content/scientist_engineers_guide/fca.html

PID Control

http://brettbeauregard.com/blog/2011/04/improving-the-beginners-pid-introduction/, blog by Brett Beauregard. This post steps through how a beginner might code a simple PID algorithm and lists some of the drawbacks they usually encounter. Brett explains how to protect against common problems like derivative kick, integral windup, and on-the-fly tuning.

General Control Theory

Control Theory, 2nd Edition, by J.R. Leigh.

http://controleducation.group.shef.ac.uk, a collection of good resources by Anthony Rossiter at Sheffield University. MATLAB resources, model predictive control, modeling and simulation, and others.

Spacecraft Guidance, Navigation, and Control

Spacecraft Dynamics and Control, by Marcel J. Sidi. http://books.google.co.uk/books/about/Spacecraft_Dynamics_and_Control.html?id=xQpZJMtDehQC This book is fantastic if you want to build simulations of spacecraft and use those simulations to design your attitude control systems. The book covers most of the basic satellite control techniques and gives you a starting point for your own design. Very little control theory in this book but a lot of good practical knowledge for a practicing engineer.

Fundamentals of Astrodynamics and Applications, by D. A. Vallado. http://books.google.com/books/about/Fundamentals_of_Astrodynamics_and_Applic.html?id=PJLlWzMBKjkC I use this book all the time as a reference for coordinate frame transformations and time. This books covers all aspects of satellite orbit determination and maneuvers. It’s a bit hard to read if you are just curious about the topic but it is invaluable as a reference if you are implementing these algorithms yourself.