Somewhere between five and ten years ago, my parents gave me The Illusion of Life: The Magic of Disney Animation by Frank Thomas and Ollie Johnston. It was a revolutionary book for me. Even today, I usually take a good read of it before I try to animate any movement, large or small, on my own.
Interestingly enough, I just found that there was a TV special in 1981 called "The Illusion of Life" (emceed by Hayley Mills, no less!). It's a great, and very dated, special about the process of animating at the Disney Studios. Perhaps best of all, it is full of interviews of the remaining "Nine Old Men"--and they are able to explain how animation changed from its earliest days in the 1920's to the (then) present. Below are all five parts.