The 39 Clues is an interactive and multi-platform adventure series combining books, collectible cards, and an online game where readers become a part of the story. According to Scholastic, "the groundbreaking series is the first of its kind in the publishing industry."
At the moment, The 39 Clues: The Cahill Files Book 4: The Houdini Escape is only available for pre-order on the Kindle (release date August 7, 2012). But a paperback version should be coming down the pike soon. I'll update this post when it does.
Thanks to Charisse Meloto of Scholastic for this first look at the cover art.