The Emperor's Code - Rick Riordan

As the race to find the 39 Clues builds to its explosive finish, Amy and Dan must explore an ancient culture and steal a Clue guarded by thousands of the world's best-trained soldiers. It's the most dangerous Clue search yet. As their enemies crowd in, Amy and Dan find themselves separated for the first time ever. The choice lies before them - find the next Clue, or find their way back to each other.