1. The criminal must be mentioned in the early part of the story, but must not be anyone whose thoughts the reader has been allowed to know.
2. All supernatural or preternatural agencies are ruled out as a matter of course.
3. Not more than one secret room or passage is allowed.
4. No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation at the end.
5. No Chinaman may figure in the story.
6. No accident may help the detective, nor must he have any unaccountable intuition which proves to be right.
7. The detective himself must not commit the crime.
8. The detective is bound to declare any clues which he may discover.
9. The stupid friend of the detective, the Watson, must not conceal from the reader any thoughts which pass through his mind: his intelligence must be slightly, but very slightly below that of the average reader.
10. Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.
And most writers of the Golden Age followed these rules scrupulously. The Footsteps at the Lock (the only one of his six mysteries which I have read) shows Father Knox to be not only a scholar and critic of the genre but one who could practice what he preached.
A bibliography of his detective works (from Wikipedia):
- The Viaduct Murder (1925)
- The Three Taps (1927). Features Miles Bredon.
- The Footsteps at the Lock (1928). Features Miles Bredon. (click title for my review)
- The Body in the Silo (1933). Features Miles Bredon.
- Still Dead (1934). Features Miles Bredon.
- Double Cross Purposes (1937). Features Miles Bredon.
- Saved by Inspection (1931). Features Miles Bredon.
- The Motive (1937)
- The Adventure of the First Class Carriage (1947)
Collaborative works by the Detection Club
- Behind the Screen (1930) (six contributors including Knox)
- The Floating Admiral (1931) (fourteen contributors including Knox)
- Six Against the Yard (1936) (six contributors including Knox)