Consider, for example, a case wherein a dead body is found drained of all blood.

If a reader comes upon that scenario in a horror novel, he knows at once that there’s a vampire lurking about somewhere. The cops may or may not twig to it, depending on the world the story is set in, but the reader knows the answer: the book says HORROR on the spine.

If a reader comes upon the identical scenario in a mystery novel, though . . . Well, now he knows it is definitely not a vampire, no matter what it looks like. Some psycho killer who thinks he’s a vampire is about as far as any “realistic” mystery novel will go.

In both cases, genre expectations define and shape our reading experience and color the ways in which we will perceive the events of the story.

It is only when the bastard stepchild takes the stage that real uncertainty sets in. Now we’re dealing with a hybrid form: part fantasy, part mystery. All the conventions must be called into question. Suddenly the puzzle is a puzzle again. Maybe it’s a vampire, maybe it’s a psycho, maybe neither, maybe both, maybe something else entirely. Better keep reading to find out.

“Better keep reading to find out” are the sweetest words any writer can hear.

Of course, today’s urban fantasists are by no means the first to cross the classic private eye story with fantasy and horror. Poe himself did it, with those murders in the Rue Morgue. Arthur Conan Doyle confronted Sherlock Holmes with the Hound of the Baskervilles . . . and though, in the end, the hound proves no more supernatural than Lassie, the story’s frisson all comes from the possibility that he may be something much darker and more frightening.

And then there is Robert A. Heinlein, the most unlikely proto–urban fantasist of all . . . but what else is one to make of my favorite Heinlein story, “The Unpleasant Profession of Jonathan Hoag,” wherein the mousy Hoag hires a husband-and-wife team of private eyes to investigate what it is that he finds under his fingernails when he wakes up every morning. Is it blood, or . . . something else?