Pirate Island Adventure by Peggy Parish, 1975.
The Roberts kids must solve another family treasure hunt! This time, they are staying with their grandparents at the family’s vacation home on Pirate Island. It has been many years since the family has been able to go there during the summer. For years, they rented the house out to a couple who lived there most of the time, and the grandparents could only go there when the couple left to visit relatives at Christmas. Now, the house is vacant again, and the grandparents have decided to take the Liza, Bill, and Jed there for the summer.
The last time that their grandfather spent the summer there as a child, his older brother decided to take something from every member of the family and make a treasure hunt just like the old family treasure hunt that the kids solved in Key to the Treasure. Just like that treasure hunt, other circumstances arose that kept the treasure hunt from being solved, so no one ever got back what grandfather’s brother hid. The only clue they have to the treasure now is a picture that grandfather’s brother drew. Since the children were able to solve the last treasure hunt, their grandfather is hopeful that they will be able to solve this one, too.
Unlike other treasure hunts that the Roberts kids solve, this treasure hunt does not involve codes and word games. The clues are given in the form of drawings, most of which (unfortunately) are not shown to the readers. The readers wouldn’t have a chance to solve the puzzles anyway because the objects in the drawings are things which the children’s grandfather has to recognize, places where he and his brother went as kids. This book also introduces Hermit Dan, who appears in another book.
This is part of the Liza, Bill, and Jed Mysteries series.