Goodreads is this great site that I'd call 'The Netflix for books' if only it contained elements of audible.com and let you download direct from the site.
Goodreads allows you to rate, review, and recommend books to your friends. It features a huuuuuge database of books from all genres and time periods (I haven't checked languages yet, though.) You're able to keep 'shelves' of books that you've read, plan to read, and are currently reading.
Find out what your friends like, and discover new books you didn't realize you were dying to read! Check it out at goodreads.com