My wife said she wanted an app where she could keep a list of all the books she owns, mark some as favorites, and then share the list with others so they know what she already has or what kind of books she likes, and let other people do the same. It seemed like a good exercise for me, so I built it. I originally used GoodReads API to search for and find books, their cover images, etc. but ported to the OpenLibrary.org API when GoodReads announced that they would disable their API.
Users can create an account, login, search for books, add books to their collection, and see other users' collections. Users can see the most popular books in the community on the front page. Future functionality might include being able to mark books as favorites, having a separate wishlist for each user, and adding referral links to Amazon to purchase books.