Jeanette Winterson writes about her mother:

ONE OF MRS WINTERSON'S objections to literature was that “the trouble with a book is that you never know what's in it until it's too late”. To extract the full flavour of this dire warning, “book” must rhyme with “spook” and be allowed four extra Os.

When I challenged her with her own taste for murder mysteries, she replied: “If you know there's a body coming, it's not so much of a shock.”

It is difficult to argue with this. However, a body isn't enough to make a good murder mystery. There are rules - very specific rules, set out by editor Ronald Knox in 1929:

1. The criminal must be someone mentioned in the early part of the story, but must not be anyone whose thoughts the reader has been allowed to follow.

2. All supernaural or preternatural agencies are ruled out as a matter of course.

3. Not more than one secret room or passage is allowable.

4. No hitherto undiscovered poisons may be used, nor any appliance that will need a long scientific explanation at the end.

5. No Chinaman must figure in the story.

6. No accident must ever help the detective, nor must he ever have an unaccountable intuition that proves to be right.

7. The detective must not himself commit the crime.

8. The detective must not light on any clues which are not instantly produced for the inspection of the reader.

9. The stupid friend of the detective, the Watson, must not conceal any thoughts that 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.

I read a lot of murder mysteries (see my September 2007 article) and cannot recall a single Chinaman featuring in any of them. Perhaps they were edited out.