Hi There!
Congratulations on your upcoming trip! My favorite Disney book for little children planning a trip is
Birnbaum's Walt Disney World for Kids. There are tons of pictures that should help your little princess get an idea of what she will experience. Another great option is
Guide to the Magic for Kids by Tim Foster, although this is probably a better pick for older children.
Another thought I had was letting your daughter sit on your lap while you look around the
www.disneyworld.com website. There are tons of interactive tools that let you "virtually" see and experience many park attractions, see the characters and start building her excitement for the big day!
I hope your little princess has a Disney trip as magical as she is! If it's her first trip, be sure to pick up a free "First Visit" pin at Guest Relations at the first park you visit!
Tanya