There's a famous list of the "rules" promulgated by the Detection Club in the 1920s, aimed at giving the mystery novelists of the time a set of guidelines for fair play. Certainly scholars of the language, as well as sociologists, could have a field day with some of the rules, including "No Chinaman must figure into the story." The original rules, written by Richard Knox in 1929, also prevented things like unexpected indentical twins and multiple secret passages from playing too large a role. You can see this list here.
I would like to add my own set of corollaries to Knox's guidelines. So many people ask me, "What exactly do you look for in a manuscript? What is the magic formula?" The simple answer is that there's no simple answer, but one thing is certain: While reading a manuscript, I think not only about how much I am enjoying but also about how much Readers, Mystery Lovers, and Non-Mystery Lovers will enjoy it. (There's no greater compliment for most mystery novelists to hear than, "Oh, I don't usually read mysteries, but I loved yours.") To this end, I present:
Agatho's Bill of Rights for Mystery Readers
Mystery readers have the following inalienable rights:
1. The right to be entertained. Reading a mystery should never be a chore. Even if it's dark or somber, it should hold your interest and inspire you to keep reading.
2. The right to have their intelligence engaged, not insulted. A mystery is at its heart a puzzle, a problem looking for a solution. A mystery should engage your brain and make you think, but allow you to go along for the ride if you don't want to make the effort. The limits of suspension of disbelief must be respected; characters who behave in ridiculous ways to keep a plot moving insult readers by making them complicit in something inane, poorly written, or ill-conceived.
3. The right to be tricked. Readers have a right to fair play, but more than that they have the right to expect that the writer is more clever than they are. Readers want to and loved to be tricked, and the best tricksters are usually held in the highest esteem.
4. The right to demand value for their money. Like housekeepers and landscapers, novelists perform a service. Mystery readers have the right to demand that writers and series remain fresh and/or offer the same results they have come to expect. A mystery novel is a product; the reader has the right to demand a high level of utility/enjoyability from that product.
5. The right to choose what they like and what they don't. Many mystery readers know what they enjoy reading and what they dislike. It's not our job to try to convince them they are wrong. If we publish books they want to read, they will buy them and read them. If they don't like the books we've published, it is our fault, not theirs.
6. The right to originality. While most mysteries are written to a time-honored formula, the reader has the right to something different or intriguing amid the murder and mayhem. Perhaps the difference is structural, perhaps something about the characters or setting. The reader has the right to feel that the author has done something more than go through the motions of compiling a bunch of genre expectations and shuffling them together.
7. The right to be heard: to talk and write about mysteries they've read without fear of censure. The Internet is full of stories and postings (I see them everyday) of readers writing their opinions about mysteries they've read, both good and bad. There is a tendency for many to dismiss negative discussion as cruel, especially by authors themselves, who cannot bear to hear their work criticized. Readers have the right to discuss any and all work, in any forum, in a respectful way, without having writers or agents responding to them in a nasty way.
8. The right to brevity. Good books need not be long, and they certainly need not be padded to justify a cover price. The best mysteries accomplish a great deal in 300 pages. Substantially higher page counts slow everyone, and everything, down. (The exception, of course, is those readers who love a big, substantial doorstop in the winter, or any other time of year.)
9. The right to a cover that reflects the book's contents. Many of today's covers are slick marketing pieces, but it's sort of shocking to see how many of them have no bearing on the book's contents, which (to my mind) is the equivalent of false advertising.
10. The right to be treated as the sophisticated grown-ups that they are. Some of the most popular television shows (such as Cheers, Moonlighting, and The Simpsons) have always taken as their operating assumption that the average TV viewer is intelligent--not a moron. People can handle complexity, and as publishers we should encourage (and publish) writers who take chances and try something new, not complain that there's not a "ready made" market for such a character or situation. Of course, we need book clubs, distributors, and booksellers to agree with all this as well.