So, I finally finished reading that mystery novel I'd been struggling with. You know it's a bad sign when it takes you more than a week to read an approximately 80,000 word mystery novel and when you can easily close the book after reading one seven-page chapter and turn out the light. That made me try to analyze why this book didn't work for me (I won't even ponder how it got published -- that way lies madness), and I think I've come up with some "rules" for making a mystery work. Of course, there will be exceptions because brilliant execution of something unexpected can turn a don't into a do, but I'm using this as a starting point as I explore the possibility of writing in this genre.
1) The heroine must have a very strong motivation to get involved in the investigation.
Normal people who are not cops don't generally involve themselves in solving murders. There has to be a good reason to do so, like the possibility that justice won't be served if the killer isn't found, and the killer isn't likely to be found through the conventional police investigation. This is most important in the first book in a series, before the heroine gets a taste and a reputation for solving crimes. Even more, I think there needs to be some kind of stakes for the heroine, where she stands to lose or suffer if the real killer isn't discovered -- she or someone very close to her might be wrongfully convicted or she might lose her job or something else important to her if the main (wrong) suspect goes to jail. I think this is a potential strength for the paranormal subgenre, because if the heroine has learned something from communicating with ghosts, reading vibes off an object, reading someone's thoughts or having a prophetic dream, then she may have more information to go on than the police do but she can't exactly tell the police what she knows or how she knows it and be taken seriously. One of the problems with this book I was reading was that the heroine never really committed to the case. She was kind of worried that someone she liked might be accused, but the cops never really acted like that person was a real suspect. It was more like, "Oh, it would be kind of nice to know who did it, and I guess if I get a chance, I'll find out."
2) There should be some sense of consequences for the heroine's involvement in the investigation.
While there should be the possibility that something bad could happen if the heroine doesn't solve the crime, I find it a lot more compelling when the heroine also gets into some hot water because she involves herself in the investigation -- sort of a damned of you don't, damned if you do situation. That could include earning the antagonism of the hunky cop, putting her reputation at stake, earning the enmity of family or community, or getting herself into physical jeopardy. I think this is usually the factor that keeps me turning pages on the way to solving the main crime.
3) The heroine should actively investigate the crime.
While she may accidentally stumble across information in the beginning as part of what leads her to decide to investigate, as the investigation continues, she should actively seek out clues, do searches, talk to people, do research, etc. There may be some serendipity, but it's boring if all the major clues come because of someone wandering across the heroine's path and telling her what she needs to know or because of her stumbling across things that she wasn't looking for.
4) The heroine should actually solve the crime.
Unfortunately, this doesn't seem to be a genre rule, but I find it irritating when the heroine discovers who the murderer is when the murderer confronts her. The heroine will be going along, thinking one person is the bad guy, until she ends up cornered by the real murderer. It doesn't take Sherlock Holmes to figure out that the person who pulls a gun on you and starts talking about how you're ruining everything with your poking around is the killer. I far prefer it when the heroine does figure it out herself before the murderer reveals himself. It can be a split second before, so she has just enough time to realize she's in trouble, but I really want her to add two and two for herself.
5) In a paranormal mystery, I like it when the paranormal ability has something to do with the case.
I don't know if there are rules for this in the genre, but one problem with this book I just read was that the heroine's unusual gifts were just set dressing. You could remove them from the story without changing the murder plot. I don't want the gifts to make it too easy to solve the mystery, either. The paranormal element works best to get the heroine into the case and to maybe complicate matters along the way. She may get a clue through paranormal means, but then she has to do real detective work to get something she can actually use.
I've also noticed that it's pretty common in mystery series right now to have some sort of gimmick, usually involving some hobby, so that the book includes some little extras like how-tos, project instructions, tips or recipes. In the series I'm planning, I think my heroine is going to be a doctor. I can just see the little how-to tips I could include: "How to conduct an autopsy -- start with an exterior examination of the body, checking for wounds or distinguishing marks."