The Mystery of the Folded Paper — Hulbert Footner

Fin Corveth's friend Nick is robbed, and an emerald locket is stolen. Murder soon follows, and the only clue to motive is a folded piece of blank paper. Dark clouds are gathering and threaten to envelope Nick's young ward, Mariula. Fin brings in mystery writer Amos Lee Mappin, as it becomes clear that greater events are afoot than simple murder. The Mystery of the Folded Paper was published in 1930.