Match other users who have read and rated books similar to my bookshelf and rating. Give a score of how "close" my bookshelf and rating is to the other users, e.g. that my list of books (that I've read) and my rating of them matches 86% to a certain user, and 75% to some other user and so on. Why is this algorthm so hard to develop (I can't understand why Goodreads haven't made this option avaliable years ago).