As one who reads as much about Walt as I possibly can, I can tell you that there are three that I highly recommend (no particular order) - Bob Thomas' An American Original - until recently "the" biography (and still my favorite), Neal Gabler's Walt Disney: The Triumph of the American Imagination (now considered by many to be "the" biography) and Pat Williams' How to Be Like Walt (a great read by a wonderful author on on eof my favorite subjects. Also, Michael Barrier has just written The Animated Man: A Life of Walt Disney that is also supposed to be very good. Both Gabler and Barrier take a much deeper look in to Walt the Man - the good, the bad, and the ugly. They are both somewhat hard to start and are lengthy, but they are both very good.
Can't go wrong with any of them...
Mark