People like Natalie Standiford

Countdown is the third book in the Unstoppable series of The 39 Clues franchise. It was written by Natalie Standiford, and published on April 29, 2014. The cover features a broken Mayan artifact, as the book primarily takes place in Tikal, Guatemala.

The back of the book states

"Life or Death: The clock has run out for thirteen year-old Dan Cahill. As head of the most powerful family the world has ever known, he and his older sister, Amy, have been in the crosshairs for too long. Dan and Amy have always managed to stay a step ahead of their enemies, but it was only a matter of time until luck failed them. Now Dan is hopelessly trapped and nothing can save him. Nothing except unleashing an evil into the world even worse than the enemies closing in. As Dan hovers between life and death, Amy has a terrifying decision to makes. Just how far is she willing to go to save her little brother?"