Richard Williams' The Animator's Survival Kit [1] is the standard recommendation, I believe - I've also seen animation courses recommend Preston Blair's Cartoon Animation [2]

[1] https://www.amazon.com/Animators-Survival-Kit-Principles-Cla...

[2] https://www.amazon.com/exec/obidos/ASIN/1560100842/

Survival Kit is an advanced book that dives into the deep end, The Illusion Of Life by Frank Thomas and Ollie Johnson [1] (two of the guys on a bunch of the early Disney features) and Cartoon Animation by Preston Blair [2] (countless MGM shorts) will give you a solid grounding in the basics.

The whole article focuses on expanding a list of principles of animation from Illusion.

1: https://www.amazon.com/Illusion-Life-Disney-Animation/dp/078...

2: https://www.amazon.com/Cartoon-Animation-Collectors-Preston-...

Preston Blair, "Cartoon Animation" https://www.amazon.com/Cartoon-Animation-Collectors-Preston-...