Disney's 'Tricks of the Trade'

(Link to YouTube Video) The decade of the 1930s was a pioneering era in animation. Artists at Disney Studios developed the new art form all the way from Steamboat Willie to Pinocchio.

The animators had to figure out the principles of character animation for themselves. As Disney says: "We took you into a unique schoolhouse where the pupils were their own teachers. They had to be because no one in the world could give them the answer to what they wanted to know."
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Books
The Art of Animation: The Story of the Disney Studio Contribution to a New Art This is one of the early books the Disney Studios published on animation.
The Illusion of Life: Disney Animation This book by two of the "Nine Old Men" is one of the standard reference books on the history and art of Disney animation.