The Curse of the Dead Man's Diamond by Christyne Morrell

In this middle-grade mystery, eleven year old Charlie moves from NYC to Florida only to find herself in the haunted Winklevoss Manor. To her surprise, she's not alone-she's joined by three mischievous ghosts cursed for snatching a dead man's diamond. After twelve-year-old Charlie moves from New York City to sweaty, sticky Florida, she'll do anything to get back home. Even if it involves ghosts. Winklevoss Manor, Charlie's new house, is a towering Victorian mansion famous for one thing-it's haunted. Three ghosts-Ada, Arthur, and Guff-live there, and not by choice. They're trapped, cursed for stealing a dead man's diamond. A diamond that, just like the ghosts, is still in the house. And this gets Charlie thinking. . .