This is Broken - A tribute to Seth Godin's book recommendations
Goodreads - See what your friends are reading.
Booknshelf - Social book and bookshelf discovery platform.
LibraryThing - A home for your books.
Readminder for iOS - Track your reading record and keep your favorite quotes
BookAuthority - BookAuthority collects the most recommended books on business, technology and science - as featured on CNN, Inc and Forbes