Imagine: you, along with your cousin, own a small but successful art gallery in downtown Chattanooga. One morning you arrive at work - and find your cousin's body on gallery's front steps. Shot at close range in the head, it looks like. As you wait for the police to arrive, you notice a package lying among the shrubbery.
The package is addressed to your cousin. Since it's already open, you can't resist a look. Inside, you find a very valuable Picasso - a painting which your skills as an art dealer tell you is a very bad fake. The return address is from another art dealer whom you know for a fact died years ago.
Why would your cousin buy a dead fake from a dead dealer? The police, who find your cousin's wallet gone, think the package is irrelevant, think your cousin died in a standard robbery. But two days later, when you're flipping through the society pages and see the supposedly dead dealer posing with the mayor's wife, you have to wonder...
Amateur detective stories are on of the largest sub-genres in mysteries today. A fairly ordinary citizen, perhaps an art dealer, becomes caught up in a murder, perhaps one that threatens to expose a huge forgery ring. Or, two days after his boss is murdered, a computer hacker finds over two hundred encrypted files on his boss's hard drive. Think: right now, you know something most people don't. Maybe you're an expert on Eastern religions. How would that knowledge be useful in solving a murder?
If you love mysteries, you've probably already considered writing an amateur detective novel. I have. In fact, I've written six, for young adults - some with supernatural twists, some straight mysteries. I've written down a few tips I've picked up about writing about amateur detectives.
1. The protagonist must have an interesting occupation. This one is pretty much self-explanatory. Through your writing, the readers experience the protagnist's life; give the protagonist something interesting to do. The amateur detectives I've read include a washed-out folk singer, a small-town sheriff, a psychic mountain-dweller, a newspaper reporter, and a minister.
2. The protagonist's occupation must be crucial to solving the mystery. Your protagonist has special knowledge that gives her an edge over the police. If your protagonist is an expert in horticulture, but the mystery centers around forensics, the rejection slips are on the way. Your heroine isn't just up against the murder - she's also competing with a police department that has plenty of detectives and crime labs. Ask yourself why your heroine would be able to solve a murder that the cops couldn't.
3. Make it personal. On one hand, if it's the protagonist's spouse or brother that gets killed, that's probably too personal. The grief and bereavement would simply be too much. On the other hand, the protagonist must have more than a casual interest in solving the murder. As the story action rises, the protagonist will likely wind up risking her life to uncover the killer - why would she do that? Even in private-eye novels, the detective usually has some personal interest - a motivation other than money - in seeing justice done.
4. The protagonist will be a raging alcoholic. Okay, so that's not a rule, and it's cliché besides. But give the heroine some kind of personal problem that she must confront in while catching the killer. In Sharyn McCrumb's The Hangman's Beautiful Daughter, the heroine is anguished by her stillborn first child. The last thing she wants is another child. Then, as the town minister's wife, she finds herself responsible for taking care of three children with horrific traumas of their own.
5. All clues will be just that - clues. In other words, no red herrings. Throwing in false clues will annoy both the heroine and the readers. Instead, have clues which seem to make no sense - at first. In my own Night of Her Life, one of the suspects has written a story which parallels the murder almost exactly. Also, this suspect has his own Web site. The heroine is positive this guy must be the killer. But in the end, she finds out that the suspect put his story on his internet site, where the real killer read it and got the idea for the murder.
6. First Who, then Why. Early in the story, every suspect will have a motive. As the story progresses, the investigation's focus often switches from Why to Where, How and When. Generally, the heroine will correctly determine who killed Mr. X before correctly understanding why this person killed Mr. X. The killer has a deeper motive, a motive related to, but not the same as, the obvious motive presented early on. The hidden motive usually isn't revealed until the climax, when the protagonist confronts the murderer.
7. The killer will be revealed. Remember that great scene in National Lampoon's Vacation, when the family has driven thousands of miles and undergone countless humiliations, only to find that Wallyworld has been closed down? This is how the readers will feel if, at the end of three hundred and fifty pages, they find out that - jeepers! - the victim actually committed suicide, but did it in such a way as to implicate murder. There will be a killer. There will be only one killer. He or she will be caught, through the deliberate actions and efforts of your protagonist.
Finally, remember, coming up with interesting - and believable - ways to break these rules is half the fun. Just don't cheat the readers. They'll kill you.