John Dickson Carr - The Three Coffins

Dr. Gideon Fell solves two impossible murders in one of the most baffling locked room mysteries ever created. Professor Charles Grimaud is found dead in his study just moments after his housekeeper watched him greet a mysterious visitor and welcome him into the room. Yet no sign of the murderer or murder weapon can be found. The housekeeper saw no one leave through the door and the snow outside the only window remains unblemished. An equally puzzling murder has occurred outside, in the middle of the street: The illusionist Pierre Fley was walking alone in a snow-covered cul-de-sac when witnesses heard someone shout “The second bullet is for you!” followed by a gunshot. He is found dead, with the revolver that killed both Grimaud and himself by his side and no footprints in the surrounding snow but his own. It appears that both murders must have been committed by a specter―someone not only invisible but lighter than air. But if anyone can find a rational explanation, it is brilliant ama