Recorded Books. The publisher's website contains ordering information for school and libraries, and Audible.com has purchasing info and listening samples for everyone.
The audiobook is narrated by Angela Goethals, who narrated the Sisterhood of the Traveling Pants series. She has also appeared in movies such as Home Alone and Jerry Maguire and on TV series such as 24 and Law & Order. I've listened to the sample and think it's amazing.