This a rather useful service by Google. Presently it offers a way of managing several public and private lists of books, called bookshelf. Google seems to have plans to add the ability to buy books. Currently Google offers some free e books and limited previews for most books.
However Since they have pretty much every book in the planet in their archives, its a great tool for just tracking the books I have read and want to read.
There is also a good number of Reviews, both User and Editorial.