Shatterproof: The 39 Clues: Cahills vs. Vespers, Book 4

After pulling some spectacular heists, Amy and Dan have become two of Interpol's most wanted criminals. So when Vesper One orders them to steal the world's largest diamond, they know they're facing life in prison...or worse. But with the Cahill hostages still in peril, Amy and Dan have no choice but to launch a mission that leads them to an ancient city full of dangerous secrets. With a Vesper mole sabotaging the Madrigals from inside, Amy and Dan have to fulfill their enemy's request before it's too late. Vesper One has developed a taste for killing Cahills, and Amy and Dan aren't going to wait to see who's next.