Maigret and the Killer by Simenon, Georges (translated from the French By Lyn Moir) - 1989

"The investigation of a murder with no apparent motive pushes Maigret to look beyond the simple question of guilt and innocence.
"Maigret, accompanying his physician on an emergency call, is drawn into one of his most stubborn cases yet. The victim, the son of a wealthy perfume manufacturer, had been enjoying an odd hobby before his death: collecting human voices with a tape recorder, often in the rougher districts of Paris. But his wallet and his tape recorder have been left untouched, so the killer's motive is unclear. The absence of clues begins to exasperate Maigret until an anonymous letter reveals that he is dealing with no ordinary criminal. " .