One of the sub-genres of the classic mystery genre is the “locked-room” mystery. In its most basic form, a person is found dead from violence in a room that is closed from within, and admits no egress. Though the person has clearly been murdered (there is no murder weapon in evidence), it seems impossible that anyone could have gotten to the victim to kill him (most times, the victim is male). The appeal of such mysteries is as much, if not more, on the ingenious solution to how the murder was committed as on the identity of the killer. We might call these “howdunits.”
The admitted master of this subgenre was John Dickson Carr, who wrote several mystery novels that might be classified as “locked-room” mysteries.
In 1935's The Hollow Man — also known by its American title, The Three Coffins — we have the epitome of the locked-room mystery. Not only is the book the exemplar of the type, but a whole chapter in the book (“Chapter 17: The Locked Room Lecture”) is devoted to a lecture by Carr’s main detective, Dr. Gideon Fell, on the topic of “locked room” murders. The novel is the sixth Gideon Fell novel out of a total of twenty-three.