As a recent Disney fanatic, myself, I have a recommendation. It's not a book, and not really about WDW, but I loved the documentary, "Walt - The Man Behind the Myth." It can be found on
Amazon:
http://www.amazon.com/Walt-Behind-Dick-Van-****/dp/B0001AVZWG
And it is also available on Netflix. Or it's been posted in multiple parts on
YouTube.
Books about WDW? I like the Imagineering Field Guide collection. There's one for each park (DHS isn't released yet). They are a great overview of the story behind each park and how the Imagineers went about designing them. A lot of interesting facts in these.