G.K. Chesterton, one of the founding members of the Detection Club, wrote an article titled such: How to Write a Detective Story. As is always the case with Chesterton, he expresses very valid and insightful points with irresistable prose; I highly recommend you read the article yourself.
But if you’d rather not (or if you did and you want to ensure you understand what he wrote—he does have a rather unique and intellectual writing style), I’ll go ahead and write a summary for any interested writers!
He presents five total points for consideration.
#1
The first and fundamental principle is that the aim of a mystery story, as of every other story and every other mystery, is not darkness but light. The story is written for the moment when the reader does understand, not merely for the many preliminary moments when he does not understand.
It’s true that the detective story is an intellectual game between writer and reader, but it’s not just the game that’s interesting. The climax of the thing needs to be worth playing for; if the author ends the story in a ditch with either no answers or a very boring, uninteresting one, the journey, no matter how spectacular, will suck. It really isn’t just the journey that matters; the destination matters just as much.
#2
The second great principle is that the soul of detective fiction is not complexity but simplicity. The secret may appear complex, but it must be simple; and in this also it is a symbol of higher mysteries. The writer is there to explain the mystery; but he ought not to be needed to explain the explanation. The explanation should explain itself; it should be something that can be hissed (by the villain, of course) in a few whispered words or shrieked preferably by the heroine before she swoons under the shock of the belated realization that two and two make four.
If the puzzle can’t be unraveled with a single sentence (and this ties into #1), the climax will be lacking. The truly soul-striking solutions are those that revolve around a single fact—a single truth—that we never thought of. It also ties into a golden rule of writing: that emotional impact is inversely proportional to length: writing too much will begin to drag down the emotional response from the reader. Brevity fits the sucker punch; and since the solution is the greatest climax of the detective story, simplicity is essential.
#3
Thirdly, it follows that so far as possible the fact or figure explaining everything should be a familiar fact or figure. The criminal should be in the foreground, not in the capacity of criminal, but in some other capacity which nevertheless gives him a natural right to be in the foreground.
If the criminal is in the foreground, they shouldn’t be obviously the criminal; as the cliche goes, “he seems too suspicious, and thus can’t be the killer.” The reader expects the criminal to be unexpected for good reason.
On the other hand, if the criminal’s a background character, the reader will feel cheated: there’s not enough exposure to really know them. There’s not enough reason to even consider them as important to the story.
Having them in the foreground, but for an entirely different purpose, is best.
#4
This I should call the fourth principle to be remembered […] the masked murderer must have an artistic right to be on the scene and not merely a realistic right to be in the world. He must not only come to the house on business, but on the business of the story; it is not only a question of the motive of the visitor but of the motive of the author. The ideal mystery story is one in which he is such a character as the author would have created for his own sake, or for the sake of making the story move in other necessary matters, and then be found to be present there, not for the obvious and sufficient reason, but for a second and a secret one.
This might also refer to the “theme” of the story, or the lasting impact of the importance of the murder. It’s even better if you can tie the two together—to have the trick of the thing both simple and related to this “theme” or reason. This one’s more abstract, but it gives the story a deeper, lasting imprint on the reader. It’s also the more “human” aspect of the mystery—why someone committed murder and what’s to be learned from it.
#5
Lastly the principle that the detective story like every literary form starts with an idea, and does not merely start out to find one, applies also to its more material mechanical detail. Where the story turns upon detection, it is still necessary that the writer should begin from the inside, though the detective approaches from the outside.
In short, it’s better to begin with a consideration of these five points and find an idea around which to base the narrative. If you just start writing with no idea, you’ll find yourself hitting the same wall: what simple twist do I want the story to turn round? What kind of killer do I want to write? How does this tie together?
These are the hard questions, but also the ones most helpful; they and actual character creation are the ones that’ll decide where the story goes and how you write it.
That’s G.K. Chesterton for you! Feel free to discuss his points (or my hopefully good summary of them) below; in addition, please check out Mystery Fiction Discussion for more on mystery fiction.