The Secret of Bete Grise Bay
"Sam and Becky are sure they can help their new friend Tom Adams - well, pretty sure. After all, Tom is a ghost who has been dead for about 100 years.
Sam and Becky and their mom and dad are visiting Michigan's Keweenaw Peninsula during summer vacation. While Mom and Dad work on their book about lighthouses, Becky and Sam help Tom solve riddles.
A long time ago an evil witch put a curse on Tom. If he can solve three riddles before the next full moon, Tom can break the curse. Sam and Becky helped him solve the first two riddles, but the full moon is tomorrow night and time is running out."