LibraryThing

Site very interesting because it's an online service and a tool to join lover's community of the book.

"LibraryThing is an online service to help people catalog their books easily. You can access your catalog from anywhere—even on your mobile phone. Because everyone catalogs together, LibraryThing also connects people with the same books, comes up with suggestions for what to read next, and so forth. "

There is in France a site similar: "L'agora des livres"