I've read a lot of good animation books. But the most helpful, in terms of actual self-improvement, was from reading and DOING the (free) "$100,000 Animation Drawing Course" on the ASIFA Hollywood Animation Archive site:
http://animationarchive.org/ (scroll down - it's on the right column). It's a series of practices from John K's blog, based on Preston Blair's classic book. I had the book as a kid, but I didn't know how to use it until I read this. It takes a lot of time but I did get a lot out of it.