*Disappears with phone for months*
Goodreads
What it costs: Free (iOS & Android)
What it offers: A community of 40 million readers to interact with; a database of 1.1 billion books to track and review; a social feed to see what your friends are reading and loving; and a fun, streamlined way of organizing and celebrating your literary life. If you aren't on Goodreads, what are you even doing? (For Windows Phone users, Epiphany and Social Reads are unofficial apps that use the Goodreads API.)
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22 Amazing Apps That Will Improve Any Book Lover's Life