Be sure to post here when you go live, I want this! I used Delicious Library to keep track of my books but it seems abandoned. I guess the challenge is to have an extensive catalog/API that makes it easy for users to add books to their library.
Haven't looked into this for a while, but most projects I saw were using Amazon's API, which I guess has the largest international catalog of books. The typical next result is GoodReads, which would be awkward to lean on for building a competitor.