Shelfari gathers book lovers from all over the world and encourages them to share their literary opinions and passions with each other. The users can build virtual bookshelves of the books they have read, and can rate, review, tag, and discuss their books. Besides that, they can make money online by linking to the books for sale at Amazon.
Something that you need to be aware of : Shelfari automatically ticks all your contacts when you want to invite someone, so if you do not pay attention, everyone in your contact list might ended up receiving invitation from you. (Source #1 & #2)