You might also be interested in renting the documentary "Walt Disney: The Man Behind the Myth". You'll find it on Netflix. Also check out YouTube for "Walt Disney: Secret Lives". Here is part one:
Here are two more that haven't been mentioned. The second book is actually a companion piece to the "Walt Disney: The Man Behind the Myth" documentary.
This is a pretty good book (it's actually called "Walt Disney: Triumph of the American Imagination"). I can recommend this one, too -- although I think the author hurries through Walt's final years a bit. It's still a very good read, though (but it's not even close to the best book I've ever read).