I signed up with LibraryThing ages ago and then forgot about it. I described it as “del.icio.us for books”, and I still think it captures the essence of the site. Like Amazon’s recommendations service, LibraryThing can suggest new titles based on your own library and overlap with other readers.
For the academic reader, how about browsing the library of Thomas Jefferson? Volunteers are adding the libraries of historical and literary figures, with the group I See Dead People[‘s Books].