Two other book sites

Under the Shelfari post, Avron says:

LibraryThing is also a book reading / cataloguing site. In what I gather is an unusual move they combine all 'editions' of each book together so it doesn't matter if you've got an e-book, paperback, hardcover, advanced release or whatever, you still connect to everyone that you could have a conversation about that book with.
They have developed a number of other neat things as well, my personal favourite is the "Common Knowledge" section, mainly because it's allowed linking of series of books together in complicated ways.

GoodReads is another book site. I'm not familiar with it however, apart from knowing it's got a lot more users.