If I buy a book it's because I REALLY want to read it, so it doesn't hang around long. The freebies are a bit different, they're there for emergencies (or when I can't be bothered to go to the library!).
Because of the BoB gift certificate offer I have a couple of books I haven't read yet, but that's really unusual for me. I generally read library books or swap with family & friends, and since I got my reader I've gone back to reading classics, so I don't spend lots on books.