For authors it is a MUST to be a member of these communities and a free (other than your time) way to introduce your book(s) and show your book titles.
BookTalk: Here you will find an online reading group and a book discussion forum that can help you discover new books (yours?).
They write: “If you’re an author or publisher looking to promote your books we welcome you to use BookTalk.org as a tool to reach a vast audience of book lovers. We have a Google Page Rank of 5, over 14,000 members and close to 700,000 site views per month.
Our book discussion forums are free and open to anyone including authors and publishers. Create a free account and tell us about your books in either the Fiction Book Forum or Non-Fiction Book Forum.
Mail a few copies of your print book and it will be advertised. We will advertise your book on our Home page for 15 to 30 days and on our Featured Book Suggestions page permanently. Mail your books to the below address. Once we receive them you will see your books advertised within a few days. You don’t have to email us to ask if we want you to mail a few copies. Just ship them and wait for your ad to appear. The system works flawlessly and is easy for both of us.”
However, there are way more book communities:
Wattpad: Join the Wattpad community to read, vote and chat with readers and writers for free. If you are a writer but have not yet published a book, you may want to check out the writing section of your profile where you can post your writing for others to read and review.
Goodreads: The Goodreads Author Program is a completely free feature designed to help authors reach their target audience — passionate readers. This is the perfect place for new and established authors to promote their books. Use Goodreads also to get book recommendations for your own reading, keep track of what you’re reading, and socialize over books.
Scribd: Find out what others are reading on Scribd.
BookMooch: Exchange books with other members.
Shelfari: Build a virtual bookshelf, see what others are reading, and discover new books.
Reader2: You can find new books to read and put your reading list online with Reader2.
aNobii: You can create and find book lists within this reading community.
Listal: Here you can list, rate, and discover books and lots of other media.
WhatsOnMyBookshelf?: This site offers readers a way to trade books with a simple point system.
Revish: Read, review and share books with other Revish members.
KindleBoards: Authors are always welcome to post about their books in the Book Bazaar.
Book Communities are a great way to kick-start your online self promotion, and get involved with a community of like-minded individuals at the same time. However don’t join to only promote your book, but to discover what other authors write and recommend them. Give & Take!
Savvy Writers & e-Books online 