I love the Official Guide to WDW, the Birnbaum guide. It's very informative, plus there are coupons in the back of the book! Make sure before you buy it (if you do that is,) that the coupons are still in the back of the book.
I got 3 recent ones from my library to see which I liked best to decide if I should purchase one. They all had similar information but it was interesting to see where they varied on the info they provided. Basically, they are all good, but if I had to choose one I'd pick Birnbaum.
But I decided NOT to buy one - I just rechecked out the library copy of Birnbaum 2007 to bring on the trip and keep in the hotel room in case I need to refer to something and will take very good care of it It's due back at the library the week after we return.
I DID buy a "WDW For Kids" book from the bookstore because it is at a nice/challanging reading level for my 7 year old, gives info on all 4 parks and he'll enjoy reading it, taking it on the plane, writing in it, etc.