A. E. W. Mason 
The Prisoner in the Opal 

The scene is the south of France. An English lady has been murdered and a beautiful American girl has disappeared. Discovered is a body with a severed hand and an opal bracelet somehow connected to devil worship. Clearly a case for Inspector Hanaud or the Sûreté and his English friend Mr. Ricardo. Can Hanaud solve the two mysteries in time to prevent a second murder? Readers will be kept in a constant state of mystification until the surprising denouement.