The Three Coffins - John Dickson Carr - Bog

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 amateur sleuth Dr. Gideon Fell.
In a 1981 survey of mystery experts, The Three Coffins (called The Hollow Man in the UK) was voted the best locked room mystery of all time. It is also celebrated for a scene in which Carr's iconic detective Gideon Fell delivers a speech expounding upon the dozens of methods and variations by which apparently impossible murders may be accomplished. Any fan of locked room mysteries and impossible crimes should consider this book required reading.