Users join a group of 6 and rotate books each month...so you contribute your own book and read 5 others that are sent to you each month. It's pretty cool...and it's free. So far I've read some great books.
I hype this site pretty often - it's an e-mail book club (actually, a dozen of them, and you can sign up for as many as you want) that makes weekly selections and e-mails you daily excerpts. It's a convenient way to sample books, and I've found a lot I wanted to read further after getting that taste.