Though many of my posts deal with technology tools, you might think I have an aversion to books-- not true. If you are looking for books to read, there are many technology tools for bibliophiles.
One of my favorites is Bookmooch. Bookmooch is a site that lets you list books you'd like to give away through the mail, and in return, you can "mooch" books from others in the community. There is no money involved except the shipping cost to send books out when someone requests them. For every book you agree to ship out, you get a point credited to you, for every book you list to give away, you earn 1/10th of a point. It usually takes only one point to request a book from someone (sometimes more if it's from overseas). It's really been a great place for me to get free books for my classroom. I've even scored a really nice box set of Maus I & II, and a hardcover copy of Twisted signed by the author Laurie Halse Anderson. There is even an area for recommending books based on your past mooching.
Other book services online I've seen or played with include Good Reads and Library Thing. Both of these are like a social network around books. They have discussion areas, you can make friends and let everyone know what you are reading, have read, and plan to read. You can rate books, and recommend them to others. What's nice is these sites often integrate with other services. For example, you can get widgets for you blog or web page that show people what books you are reading, what you like to read or what books you are looking for.
In addition, Amazon lets you sell your used books through them and friends of mine had great luck with that. You can buy used books there as well (they are listed along with the new ones for sale in your search results).
If audio books are more your style, try librivox. This is a community, volunteer-driven, that is dedicated to creating free audio files of literature, non-fiction, drama and poetry. They only have works that are no longer under copyright protection, but it's a great way to hear some great writing, and you can volunteer to record a work yourself. I contributed a small poem about a year ago, "How Doth the Little Crocodile" by Lewis Carroll and it was fairly easy to do. A great way for kids to practice public speaking... to a real public.