The 39 Clues Book 2 One False Note

When Grace died, Dan and Amy Cahill's life was turned upside-down. Not only had they lost their closest relative, one whom they share endless memories of, but they were also catapulted into a global treasure hunt with little more than a few cryptic clues to guide their way. After discovering that the Cahill family line extends to include nearly every significant figure in history, Dan and Amy question where they fit in. But time is short and there's not a moment to waste as they travel to Mozart's homeland in hopes of solving another clue.