Currently, I'm reading the Neal Gabler biography of Walt Disney.
It depends upon what facet of Disney you're interested in. I've read all the aforementioned books.
If you're interested in animation, there's "Disney Animation - The Illusion of Life" by Frank Thomas and Ollie Johnston.
If you're interested in the theme parks, besides "Designing Disney", there's the books by Jason Surrell and the various Imagineering Field Guides and "Walt Disney Imagineering" by The Imagineers.
There are many biographies on Walt Disney. The company's official biography is "Walt Disney - An American Original" by Bob Thomas. For a persepective of Walt from those who worked closely with him, there's "Workiing with Walt" by Don Peri.
If you're interested in the creative people Walt gathered around him, there's "Walt Disney's Imagineering Legends" by Jeff Kurtti.
I hope this helps you start building your Disney library.