There’s more to animation than the 12 principles, so I’ve been doing less reading on animation itself and more in other related areas.
Just click on the images to find out more, I’m not the greatest of book reviewers. All I’ll say it The Visual Story is an absolute must have, Invisible Ink is a great (Kindle) bargain, still reading Story and Shot to Shot.
I think you’ll find these on a lot of recommended reading lists. A lot of people also recommend The Artist’s Complete Guide to Facial Expression and Emotions Revealed when recommending books about understanding the face. The Emotions Revealed book is a very interesting read, but I found them both to be a bit too detailed for learning how to animate a face. I thought a great explanation was in Scott McCloud’s Making Comics book. He goes over the anatomy of the face and Paul Ekman’s research in a super concise and visual way. If you’re lucky you might be able to google for those specific pages.
Another common one is Acing The First Six Lessons. Again I don’t highly recommend it, I think body language has more relevance to what we do than books on traditional acting.