One thought is a social networking thing where your digital library is visible, but only those parts of it which the producer certifies that you've paid for.

Do people really use sites like that as some sort of book epeen? I like librarything as a way to organise what I've read (and need to read) and publish reviews, not because I like showing off how many books I "own". My goal is to own no books, so the idea that someone might want a site that only shows books you own, not just any books you've read is really weird. (And I would hate it if librarything, etc. used that model, since they would then be useless to me.)

(Oops, realised afterwards you just only said digital there.)