Link: Google Books.
Check it out! You can create your own personal list of books, complete with reviews, tags, RSS feeds, and stars in Google Book Search! You can import books by ISBN! Think of educational implications... annotated bibliographies of recommended books, book lists tagged by teaching units, writing book reviews, running records of books read, browsing lists of other people, etc. Any other ideas?
UPDATE: Here's a link to my library so far. I'm planning on starting with my favorite childhood tomes. Make sure you click on one of the About This Book links as it takes you to an interesting page with links referencing other resources.