Key to the Treasure by Peggy Parish, 1966.
When twins Liza and Bill and their older brother Jed visit their grandparents for the summer, their grandfather tells them the story of a treasure hidden somewhere on their farm. Over a hundred years ago, their great-great-grandfather was friends with an Indian (Native American) woman. When she died, she left her belongings to him, including a feathered bonnet, a leather shield, a mask, and a doll.
When he grew up, he went away to fight in the Civil War. Before he left, he hid three of the items (the shield, the mask, and the doll) and made up a series of clues to help his children find them. Unfortunately, the first clue was lost, and he never returned to tell his children where he hid the treasure. The only clue that was left was a picture of the feathered bonnet, a little pot, and a strange-looking key. From then on, each generation in their family has tried to solve the puzzle. Since so many generations of their family have failed to find the treasure, Liza, Bill, and Jed don’t have much hope for success until they accidentally stumble on something that gives them their first clue.
Together, the three children quietly work on the clues, following each to the next. They hope to surprise their grandfather with the solution to this old family mystery. But, can they?
The clues to the treasure take the form of word games and codes (the first is a simple substitution code, and the other two are word games). Young readers will find it challenging to solve the codes along with the children in the book (Liza, Bill, and Jed walk readers through their solutions as they work on the puzzle, even showing how they correct for mistakes along the way), although older readers may find them rather easy. It’s a good book for introducing children to codes and word games if they haven’t really encountered them before. For the challenge of it, you can stop reading at each new puzzle and attempt to solve it before the kids do!
This is the first book in the Liza, Bill, and Jed Mysteries series.