I found out about this site while researching Open Social last month.
While it's no way as sexy as Delicious Library it's web based and I can use it on my dirty PC. Delicious Library is Mac only. Boo!
The one feature missing which is on Delicious library is "loan" which lets you keep track of who you have loaned your books to. Which would help me as I don't know who has my Zombie Survival Guide!
The appeal of tracking all the books you have read and own is two fold. One you get to show off and two you will discover new books. Either by the same author or by looking at your friends books.
It also says a lot about who you are by what you read.
Librarything (the biggest in this market) let you know what they think Shelfari and how they have been spamming users.
Library Thing doesn't look as sexy but the huge user base is appealing. But I've just spent an hour adding all my books I can;t be assed doing it again.