Well, I own at least one copy of about all the books:
Unofficial Guide; Passporter; WDW for Couples; WDW and Orlando for Dummies; a Rita Aero book; and Birnbaums.
The only books I get a new copy of every year are Passporter and the Unofficial Guide.
The Passporter is great to have on your trip--it makes organizing very easy and is light enough to throw in your backpack to carry around the parks. Plus their maps are great and have the parade routes for each park outlined.
I LOVE the Unofficial Guide. Bob Sehlinger's writing style is very conversational, and he has just enough of the sarcastic and cynical edge that I enjoy. Besides, I can always find something new that makes me howl with laughter in each addition. I always take this book, too. I read it cover to cover each year, review my favorite parts on the plane ride down, and then leave it in my room.