There are a lot of books based on animation and its forms; 2D, 3D, stop-motion. We listed what we believe is the top 3 below!
1) The Animator's Survival Kit by Richard Williams
This book is considered the the animation bible to many. Most universities make it a required textbook. It's an easy read with great in-depth examples of all the principles of animation. It will take you step by step through Richard Williams's animation techniques and will get your mind ready to think as an animator. Watching someone walk down the street will never be the same again. After reading this book, you will be paying attention to how long their stride was, do they land on their heel or toe, how "bouncy" is their walk, and so much more.
If you are drawing a blank on who Richard Williams is... He is the academy award winning director for "The Pink Panther", "Who Framed Roger Rabbit", the 1971 "Christmas Carol", and many more. In other words, he knows what he's talking about and you can have direct access to his thought process with this book!
2) The Illusion of Life: Disney Animation by Frank Thomas and Ollie Johnston
Disney is in the name which means you are already expecting it to be magical. It was written by 2 of Disney's "Nine Old Men", Disney core animators when the company first began and defined the Disney style. This i their in-depth journey of how they explored the principles of animation and really pushed it to get the Disney look. A great history as well as technical examination on the subject.