I enjoyed Neal Gablers "Walt Disney:Triumph of the American Imagination". All of my reading is done via audiobooks, and this is the only biography available on audio. It is over 30 hrs of audio and a exhaustive work on Walt's life.
I've read several different books about Walt and Gabler's book is my second favorite. I think the best by far is Bob Thomas' Walt Disney: An American Original. It is well recognized by many as the definitive work on Walt - and yes, there are many that would (and do) argue Gabler's is the best. Can't go wrong either way - better yet, read both and decide for yourself