Servo Anatomy, In Depth

A really great presentation.  He covers the usual PWM stuff (nicely), and then talks about the electronics inside the server, including how the controller works.  It took me a couple of weeks of self-study to get about 75% of the stuff he covers.

There's lots of other interesting stuff on the site as well.

Supplement: Controller magic is courtesy of the Mitsubishi M51660L.  Another great writeup is here.