You'll note that some of these are dead or dying, which leads me to the small point that I'd make: creating a book site is pretty easy: the audience likes print-media and there are rafts of digitized content available, but getting a site to sustain is hard.
http://www.librarything.com/ http://www.shelfari.com/ http://49thshelf.com/ http://openbooktoronto.com/ http://www.filedbyauthor.com http://www.thecopia.com/home/index.html http://www.booksprouts.com/ http://www.bookglutton.com/ http://weread.com
You'll note that some of these are dead or dying, which leads me to the small point that I'd make: creating a book site is pretty easy: the audience likes print-media and there are rafts of digitized content available, but getting a site to sustain is hard.