Mr. Summerling's Secret Code (The Treasure Troop, Bk. 1)

Marly always knew Mr. Summerling as her friendly neighbor living in the big, old house next-door. Sure, he walked around with a metal detector and talked about being a "treasure hunter," but she didn't think much of it. But when news of Mr. Summerling's death arrives at her doorstep, Marly is brought into a treasure hunt of her own. In Mr. Summerling's will, he's left a treasure for Marly and her two classmates, Isla and Sai. The catch? They have to solve a series of riddles, puzzles, and clues to find its location. And not only that, they have to work together on it -- which Marly is not looking forward to. But with no other choice, she, Isla, and Sai set off on the hunt. Can the three kids come together to crack the code? And even if they do solve the clues... what could Mr. Summerling possibly have left them?