I bought a copy of walt Disney world with disabilities. A few months later I was on their site and saw that the book had been revised and they were offering free copies to anyone who bought it before then, so I got one. On the site they said that they had hired Disney employees to bring the book up to date and make corrections. They also added a lot more information.
On the web they mentioned that this was their first book, and there was a learning curve. I think it was a stand-up thing that they did this. I used the revised book on my trip, and I found it to be accurate and oh so helpful!
I also have Passporters, which is excellent, but WDW with Disabilities is excellent too, but just different from Passporters. There was a lot of really useful stuff in there that wasn't in Passporters, and vica versa, so I think they go great together and would recommend buying both.
I originally bought wdw with Disabilities because I saw it discussed on this forum. That thread had a lot of people on it that really liked it, and thats why I bought it. Considering that they made it accurate now, I guess it's a matter of taste and opinion at this point. I'd prefer to have as much help as I can get, and I was glad that there's more than one book out there that really helped me.