He Who Whispers — John Dickson Carr

A DR GIDEON FELL MYSTERY. Outside the little French city of Chartres, industrialist Howard Brookes is found dying on the parapet of an old stone tower. Evidence shows that it was impossible for anyone to have entered at the time of the murder, however someone must have, for the victim was discovered stabbed in the back. Who could have done it? And where did they go? When no one is convicted, the mystery remains unsolved for years until a series of coincidences brings things to a head in post-war England, where amateur sleuth Dr. Gideon Fell is on the scene to work out what really happened.