I buy a Passporter for every trip....I use it as a trip diary. I buy the old ones for my dd, 12, so that she can have her own trip diary (you can buy the out of date ones on-line for quite a bit less). I also buy the UoG every year, just to compliment my Passporter. And sometimes I will even buy Birnbaum's. I do buy the Birmbaum's for Kids for my dd also....you can't beat the photos in the Birnbaum books, since they are the 'Official' book for Disney.
You will have a much more successful trip if you get at least one guidebook before you go. Otherwise, you won't be fully prepared for the huge energy sucking thing that WDW is! It is so worth the cost of those books....you will know what you want to do, what you don't want to do, and the best way to get done most of your must-do's. Happy planning!!!