GoodBooks.io - Largest curated collection of 8,500+ book recommendations.
Goodreads - See what your friends are reading.
Read This Twice - Verified book recommendations from people we look up to
LibraryThing - A home for your books.
BookAuthority - BookAuthority collects the most recommended books on business, technology and science - as featured on CNN, Inc and Forbes
Oku - Track your book lists and get recs from your friends