The Passporter and the Unofficial Guide to WDW.
Passporter is an easy read from cover to cover, and is great to take with you on the trip. I always suggest it as a starting point for people who are new to WDW because it is not overwhelming. The Unofficial Guide is like the Disney Bible. It is a great reference book, and has just about everything you need to know to plan a trip.
If I only had to suggest one, I'm not sure which I would pick. I think if the person was totally new to WDW, then I would say Passporter. If they had been to WDW before, I'd say the Unoffical Guide. But really, I think anyone wanting books to help plan a trip should get both of them. But I think the Passporter would be my personal favorite because I really like the ride/attraction/restaurant descriptions and the colored fold out maps of the parks.