This is book 5 in the Ghostly Graphic Adventures series.

Joey and Tank take a trip to explore Star Island, one of the Isle of Shoal in New Hampshire. Perhaps they’ll find some of Blackbeard, the pirate’s treasure. While on the island they meet up with Mitty who lives there, only to discover later she may be one of the island’s many ghosts.

On page 31 there is a mini history of Star Island.