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.
Cartoon Animation, Preston Blair (Walter Foster Publishing, 1995)
Good supplemental animation book from one of the traditional masters, this book is chock full of animation breakdowns, both human and animal.
How to Draw Animation, Christopher Hart (Watson-Guptill Publications, 1997)
The Human Figure in Motion, Eadweard Muybridge (Dover Publications, 1989)