Don’t miss this rare find on Archive.org: A complete PDF copy of John Halas and Roger Manvell’s book Design in Motion (1962). The book is filled with inspiring stills from both famous and obscure Cartoon Modern-era films that you’ll want to see. I talked a little bit about the book’s value to me in this post about animation reference books.
Also on that site:
How to Cartoon for Amateur Films by John Halas and Bob Privett
Animation Art in the Commercial Film by Eli Levitan
A 300-page oral history with Jules Engel
A 250-page oral history with Dick Huemer.