I have just stumbled upon a new social network dedicated to books - Shelfari
It was launched in October 2006 and on the 28 February they announced
that they have Amazon among the funders.
It seems quite promising, but I think that whether it succeeds or fails remains to be seen since it (as many of the social web 2.0 startups) relies on large user base to be fully able to deliver what people would like to see from it. They don't give the number of users, but the biggest group there has 114 users, which is not so amazingly high for almost 5 months in function, but I hope that with the publicity it received now thanks to the mentioned announcement the number will rise more quickly.