BoomWriter is a free service that let's students write a book collaboratively. They are now offering to publish the book for free and ship a free copy to your class. This offer is for all teachers in the United States.
They have also made some updates to the site and teacher dashboard to make it easier to use.
Here's a video that explains BoomWriter in more detail and shows how it can be used to help students read and write better.
Check it out: http://www.boomwriter.com/schools