It costs nothing to join and the only out of pocket expense is shipping, but there's a 5:1 ratio: you have to send out at least 1 book for every 5 you receive. The system works off of a point system:
Action ~ Points
Add a book to inventory ~ + 1/10th
Remove a book from your inventory ~ - 1/10th
Give away book (within your country) ~ + 1
Give away book (to another country) ~ + 3
Mooch a book (within your country) ~ - 1
Mooch a book (from another country) ~ - 2
Acknowledge receiving a book you mooched ~ + 1/10th
You search for a book you want by title or topic, and if it's not available you can make a wish list and the site will you know when your requested books become available. You can also donate your points to a charity so that someone needy can recieve a book.
If you love to read the way I do this can save some money, plus maybe introduce us to new authors.