Addie McCormick and the Secret of the Scarlet Box (Addie Mccormick Adventures, 7)

Addie and Nick's discovery of an old lockbox sends the two young detectives scrambling on their most intriguing case yet. They learn that the box is part of a treasure hunt begun 50 years earlier and Nick and Addie join forces with another group of children to find more clues. But soon the competition gets out of hand.