Lyndaman Island Manor — Kathleen W. Forbes

Lyndaman Island Manor is the fourth book in the Holly Brannigan Mystery series. Holly Brannigan and her three teenage friends, Paul Castles, Ted Lumley, and her best friend, Bonnie Tilson, find themselves in a dangerous situation when they visit Lyndaman Island Manor at the invitation of multi-billionaire Harvey Fields. They invite eight friends from their scuba diving club to come with them.

Strange things begin to happen after they arrive on the island, so Mr. Fields calls in Holly's father, a well known detective, to solve the case. Holly and her three friends had helped Detective Brannigan in the past when teenagers were the culprits. The four young sleuths were able to infiltrate teen groups to get information. There are dangers and threats at every turn on the island.