Hi Julie,
Great question! In alphabetical order, my favorites are:
"Birnbaum's Walt Disney World"
"PassPorter's Walt Disney World"
"The Complete Idiot's Guide to Walt Disney World"
"The Complete Walt Disney World"
"The Unofficial Guide to Walt Disney World"
I have them all next to my computer as I answer e-mails. They are all good in my opinion. However, I suggest going to a local bookstore to review them and see which appeals most to you. While they all cover Walt Disney World, their approach can be very different.
As an engineer I love all of the nitty-gritty details that can be found in "The Unofficial Guide". However, it doesn't have a lot of photos. If I want to "wow" a friend or neighbor into planning a first trip to Walt Disney World, then I usually give them "The Complete Walt Disney World". The 2009 Edition was filled with over 400 photos and was spectacular to look at!
For U.S. and Canadian household, you can order a free planning
DVD. Have fun planning your trip!