There are some books in the scrapping store connected to The Art of Disney. It used to be just a bookstore but now carries mostly for scrapping materials. I can't recall the name! We found some very interesting books about Disney (I bought one about the Haunted Mansion and another for DS about hidden Mickey's) but I'm not sure if they had just regular books.
Another possibility might be the huge Virgin Records store on the West Side. I haven't paid attention while in DD but I know that other Virgin Records stores have book sections.
There also used to be a bookstore in MGM and maybe someone can confirm if it's still there.