GoodBooks.io - Largest curated collection of 8,500+ book recommendations.
Readwise - Effortlessly rediscover and organize your Kindle highlights
BookAuthority - BookAuthority collects the most recommended books on business, technology and science - as featured on CNN, Inc and Forbes
Clippings.io - Organize the notes you make on your Kindle
Goodreads - See what your friends are reading.
Klib - Kindle & iBooks Highlights Manager