"Haunted Island" is a young adult horror/mystery novel by Joan Lowery Nixon, who wrote somewhere over 100 books in that category, with much critical and commercial success. It was first published in 1987.
Amy and Chris Holt are two middle school age kids, staying with their aunt at a bed and breakfast she runs in Missouri. She wishes to turn an island in the Missouri into a tourist attraction, but according to the locals, the old house on the island is haunted. (Incidentally, the who, where, and what are all laid out in the first few pages---something that is sometimes a relief for a reader). Amy and Chris wonder how to help their aunt, when they see a strange neighbor on the porch--- who proceeds to tell them the history of when he lived on the island, when it wasn't an island, with an angry, miserly settler and his suffering bride. After an earthquake strikes, the house is left on an island, and the owner, Joshua Hanover, becomes a ghost. When the children report this story to their mother, she says it is impossible, because that earthquake was the great New Madrid Earthquake, which took place almost 200 years ago!
Hmmm...who could this mysterious old man be?
Nevertheless, they let the old man row them across the river to the island...only to see him fade away before their eyes. They then spend the remainder of the story running to and fro across the island, trying to find out what secrets Joshua Hanover, his bride, and his servant had. By collecting the right items in the right places, they hope to put the ghosts to rest...and return to their home across the river. At this point, it becomes more of an adventure story, as they run across the island, dodging its natural hazards and the angry ghost of Joshua Hanover. They eventually solve the mystery and leave the book with a happy ending.
In general, I liked this book, but it fell victim to a common problem among works of horror---and not just ones for younger readers. In trying to explain and resolve the central conflict of the story, it had to dispel with the mystery and the sense of the eerie. Once we know about the motivations and basic rules about ghosts, the story is a straightforward quest to achieve goals while avoiding obstacles. At first, I was thinking the story might have a bit of the Scoobie-Doo in it: perhaps the old man was just pretending to be a ghost to protect his treasures. But nope, no hint of any type of deception or magical realism here, just some very broad hints of the supernatural, a single revelation that ghosts are real, and then two kids running around an island on a scavenger hunt, followed by a plot resolution. It makes sense, many stories of the supernatural fall victim to the same rulification of something mysterious, but I still found myself a bit disappointed, despite the story being well-written and easy to follow.
Apparently, from what I have read, Nixon's other works were more likely to play with the mysteries in them, and I would certainly be interested in reading them, if given a chance.