I was working on a book social site where users earn badges after reading books. (Basically Foursquare for books. There would be badges for reading the top 10 books of a genre/author, milestones, etc.) Gave up on getting a good book database.
Sounds like an interesting idea - I love reading, and a lot of ideas I've had have been book-related. Just curious - why not utilize Amazon's book database, since that is likely how you'd make your money anyway (via referral fees)?
I started using Amazon, but when I started thinking about adding badges and it would need the book database to be very specific about authors/genres/titles, (which Amazon's wasn't) I realized it was way out of my league.