As I read through it, I thought about the different kinds of mystery novels and sleuths. The book mentions some traditional genres of mystery: the hardboiled thriller (for instance, Mickey Spillane's Mike Hammer mysteries); the science-fiction mystery (Isaac Asimov's Elijah Bailey and Douglas Adams' Dirk Gently stories come to mind); the cozy mystery (Joanne Fluke's Hannah Swenson mysteries are good examples of this). There are other categories, too, of course.
Myyteries and sleuths don't always fit neatly into one or another categeory. I'm sure you could think of any number of mysteries you've read that aren't, strictly speaking, in one genre. I know I can. What I find interesting is the difference between mysteries that feature sleuths who are already in the law enforcement/detection busienss and those who aren't. Some mystery series (Kevin Hughes's novels, for instance) feature a sleuth who is (or was) a police officer. Others (Michael Collins' Dan Fortune is one example) are centered on private investigators - again, very close to the law enforcement business. Janet Evanovich's Stephanie Plum series and Sue Grafton's Kinsey Millhone series are other examples of series that feature what I'll call deliberate mysteries. The sleuths are in the law enforcement business, or a related business. They expect to solve mysteries. One might even say they look for it.
Other mystery series are what I call "accidental mysteries." By that I mean that they feature sleuths and other characters who aren't in the law enforcement business, or anything close to it. They have other jobs, or they have other hobbies. They certainly don't start out by looking for crimes to solve. They run into mysteries because they're in the wrong place in the wrong time, or because, like Laurien Berenson's Melanie Travis, they're curious. Dorothy Gilman's Mrs. Pollifax got into the business of solving mysteries because she was bored living in retirement. Mary Jane Latsis and Martha Henissart (pen name: Emma Lathen) created John Putnam Thatcher, senior vice president of the Sloan Guaranty Trust bank. Thatcher gets involved in mysteries as the bank gets involved in different industries and events; Murder to Go and Going for the Gold are two examples of this. Perhaps the most famous example of the accidental mystery can be found in Agatha Christie's Miss Marple series. Miss Marple isn't a detective (although her nephew is). Yet mysteries happen to her. People she knows get involved in murders and other mysteries. She is drawn into the events around her.
Accidental mysteries fascinate me because the characters in them don't really have access to the machinery of the law that's available to characters who are involved in law enforcement. How do you solve a mystery and figure out who committed a murder if you don't have authority to interrogate witnesses, look for evidence, and so on? Authors of accidental mysteries have different ways of answering that question. Sometimes, the sleuth has a gift for observation and knowledge of human nature; that's Miss Marple's true talent. Other sleuths are just curious and they don't give up until they find out the truth. They ask questions, poke around and get their answers. Rita Mae Brown's Mary Minor "Harry" Haristeen is like that.
My own Joel Williams series doesn't fit neatly into the deliberate or the accidental mystery category. Joel Williams is a college professor; he's not a police office and has no authority to question or get evidence. So sometimes it's a challenge for me to write about his sleuthing in an authentic way. It's also a challenge to make sure that, in making Williams my sleuth, I don't make the official investigators look ridiculous. That's one reason for which Williams is a former police officer, and still has friends among the Tilton (PA) police force. It allows Williams and the police to do what they do well, without getting in each other's way. It also gives Williams an edge when it comes to noticing things, putting clues together and getting to the root of a mystery.
What kinds of mysteries do you like? Do you see a difference between what I've called deliberate and what I've called accidental mysteries? Do you prefer one or the other?