Bibliophil.org has a lot of features including cataloguing, friends, sorting by many fields, ratings of books, recommendations, loaning of books, search for best price, wish lists, data export, mobile access, and it's all free.
Book Jetty allows you to catalog, rate and review books, check availability through libraries (from 11 different countries), discuss books with firends, display on your blog
Titlz intends to provide data and trends on the book publishing industry - historic data, sales rankings on books on Amazon, trends on topics and titles. Although still in development, book industry professionals can currently use TitleZ for free.
Shelfari is a social network for people who love books. Create a virtual shelf to show off your books (which looks like a shelf), share them with friends. Pretty site.
Books are a rich source of information. #Reading books on varied subjects imparts #information and increases the depth about the subject as well.
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