The Animator's Survival Kit, Richard Williams (Faber and Faber, 2002)
Veteran animator Richard Williams has written a veritable bible for the aspiring animator. It covers every aspect of animation and is useful for both traditional and computer animators.
Teach Yourself Body Language, Gordon R. Wainwright (Hodder & Stoughton Educational Division, 1999)
Not exactly an animation book, but having knowledge on why we do what we do is essential to any animator.
The Artists Complete Guide to Facial Expressions, Gary Faigin (Watson-Guptill Publications, 1990)
This is an exhaustive reference guide to facial expressions and the psychology behind them.