The Forbidden Tower — Frances K. Judd

Kay's club, the Pathfinders, travels to Florida for a vacation, and Ethel Eaton's rival club, the Bittersweet Society, follows the Pathfinders, causing trouble along the way. Once in Florida, Kay searches for Emily Dodd, who is missing, and investigates strange events at a nearby estate. The owner, Jasper Jackson, is unfriendly and hateful towards Mrs. Dodd, and Kay wonders if he might have something to do with Emily's disappearance. After the girls see a ghostly figure on Jackson's property, they know something sinister is behind it. Kay divides her time between her investigation, spending time with her friends, and warding off Ethel Eaton's attacks.

The girls must survive several dangerous situations and use all of their wits before they uncover the secret behind Emily Dodd's disappearance and the strange events on the Jackson property.