The best way to look more into these would be to buy the book but if you can't afford to there are plenty of places online that give their own explanations.

https://discord.gg/QW7UTdftFW - Discord Server

https://www.nyfa.edu/student-resources/12-principles-of-animation/

https://www.adobe.com/creativecloud/animation/discover/principles-of-animation.html

https://d23.com/a-to-z/nine-old-men/

https://disney.fandom.com/wiki/Disney%27s_Nine_Old_Men

https://www.goodreads.com/book/show/106731.The_Illusion_of_Life