I use Bookmooch -- very similar service. I have gotten tons of books and have sent out an equivalent number for $2-3 each. Not completely free because of the media mail postage, but a bargain. Another nice feature -- you can easily upload your Amazon wishlist into it, and they'll automatically notify you when/if books your list are offered by other moochers!
No affiliation, just a happy member: http://www.bookmooch.com
Emily
2011 Jamis Dakar XC "Toto" - Selle Italia Ldy Gel Flow
2007 Trek Pilot 5.0 WSD "Gloria" - Selle Italia Diva Gel Flow
2004 Bike Friday Petite Pocket Crusoe - Selle Italia Diva Gel Flow