Haunted Island — Joan Lowery Nixon

When their aunt buys an old, run-down inn on the banks of the Mississippi, Chris and Amy Holt go south to help her fix up the place. They lend helping hands during the day, but at night, they wonder about the island in the middle of the river-a mysterious place where the townsfolk will not set foot. The island was created by an earthquake which shook the ground so fiercely that the river ran backwards, destroying the house of a bitter old miser named Joshua Hanover, and carrying away his precious hoard of silver coins. And according to legend, Hanover's ghost still roams the island, his phantom dog, Shadow, at his side, searching eternally for his lost treasure. Unable to resist the treasure's lure, Chris and Amy journey to the island to find out if the stories are true. There they find danger, and a mystery that is far more frightening than they could have ever imagined.