Animators Survival Kit - Richard Williams
When I first discovered this book, it was like hitting the jackpot and finding treasure everywhere! Best standalone book on animation that covers all the core principles.
The Illusion of Life: Disney Animation - Frank Thomas and Ollie Johnston
Great insight to the wonders of Disney Animation.
The Artists's Complete Guide to Facial Expression - Gary Faigin
This book completely changes the way I perceive facial expressions. Incredibly detailed and full of great references. Highly recommended!