The Treasure Troop Series by Dori Hillestad Butler

Join Marly, Isla, and Sai, three code-cracking kids on the hunt for an old neighbor's hidden treasure.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... moreJoin Marly, Isla, and Sai, three code-cracking kids on the hunt for an old neighbor's hidden treasure.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? less

Join Marly, Sai, and Isla, three code-cracking kids on the hunt for an old neighbor's hidden treasure.Marly, Sai, and Isla are back on the hunt. After finding Mr. Summerling's hidden treehouse, they've discovered a new set of clues for them to follow, leading them straight to the heart of their mysterious neighbor's life: his house. The same house Marly has been living next to all of these years, which, they discover, is filled with hidden rooms and staircases... moreJoin Marly, Sai, and Isla, three code-cracking kids on the hunt for an old neighbor's hidden treasure.Marly, Sai, and Isla are back on the hunt. After finding Mr. Summerling's hidden treehouse, they've discovered a new set of clues for them to follow, leading them straight to the heart of their mysterious neighbor's life: his house. The same house Marly has been living next to all of these years, which, they discover, is filled with hidden rooms and staircases. With just a few scraps of papers as clues, and Mr. Summerling's angry son on their tails, can the Treasure Troop find the answers they're looking for? less

In the third book of this code-busting series, Marly, Sai, and Isla are back on the hunt! Mr. Summerling has left them a brand-new set of clues to follow, including tickets to the mysterious Summer Island. So with tickets in hand, our trio sets off to their late neighbor's island. Can they finally discover what lies at the end of this treasure hunt?