[n_plus_one]

This was a pretty good article for helping to learn about PID. http://m.eet.com/media/1112634/f-wescot.pdf

It's about 15 years old, but still a pretty good intro to basic control systems for people without a background in the field.

[kqr2]

If you like the article, the author Tim Wescott went on to write an entire book on applied control theory for embedded systems:

http://smile.amazon.com/Applied-Control-Embedded-Systems-Tec...