“How come you review Robert B. Parker's Spenser novels in Fiction?” Rob Rosenwald, the publisher of Poisoned Pen Press, once asked me, PW's Reviews editor in charge of mysteries and thrillers, who ought to know. “I thought he wrote mysteries.” I often get this sort of question. Why are certain books reviewed as general fiction and similar books reviewed as category mystery?

The basic rule I follow is this: thrillers (spy, legal, medical, etc.) are reviewed under Fiction; mysteries (whodunits, ranging from cat cozies to hard-boiled noir) under Mystery. To make a simple distinction: in a thriller, the heroes are in a race to save the world from known villains out to destroy it; in a mystery, a sleuth seeks to solve a murder committed by an unknown killer whose identity the reader tries to figure out before it's eventually revealed.

Sure, there are plenty of hybrid cases, but distinguishing the two should—in theory—be easy. In practice, it can be hard, since I also generally go by the labels publishers put on their galleys, labels that often have more to do with marketing than content. One can understand the impulse to call what the average reader might consider a mystery something else—like fiction or suspense—given that many thrillers sell at bestseller levels, while most category mysteries depend on relatively modest library sales.

This tension between marketing and content can confuse even publishers, including St. Martin's, whose Minotaur imprint, after years of putting out category mysteries, has lately been venturing into thriller territory. British author Anna Blundy's Vodka Neat, an excellent May Minotaur title, for example, is subtitled “A Faith Zanetti Thriller” but labeled “Mystery.” As the gatekeeper, I decided to put this one—which deserved a starred review—in Fiction because I already had enough stars in Mystery that week.

I cringe whenever I see an obvious thriller use the word mystery in a title. Next month, MacAdam/Cage is bringing out Sheldon Siegel's Judgment Day, subtitled “A Mike Daley Mystery,” a book many would consider a legal thriller. Since I was short on category mysteries that particular week, I ran the review in Mystery.

No doubt some people in the industry take their cue from mystery/thriller maven Otto Penzler, who insists that the thriller is a kind of Mystery, the category label he uses for titles under his Harcourt imprint. At the Edgar Awards last year, I asked Penzler, did he really want Joyce Carol Oates's latest story collection reviewed in Mystery. No, he preferred Fiction, and that's where I put it.

Not so long ago, Janet Evanovich's publicist politely suggested it was time Evanovich's Stephanie Plum novels were reviewed in Fiction rather than Mystery. I had to agree it was now that the series was hitting bestseller lists and made the switch. Typically, a series will start out as mystery and later get upgraded to thriller, with either increased sales or the hope of same. At some point, Walter Mosley's Easy Rawlins mysteries became Easy Rawlins thrillers.

One way to avoid this confusion, at least for my purposes, would be to combine thrillers and mysteries under a single heading, as Borders does in its stores. (Barnes & Noble maintains a separate Mystery section and sticks thrillers in with general fiction.) For the consumer, this would be as natural as grouping, say, science fiction and fantasy in one place.

Such a shift, however, would risk igniting the perennial debate over what's literature and what's genre. Many authors whose books have been reviewed under Fiction would resent being rebranded as genre, never mind Patrick Anderson's argument in his opinionated survey The Triumph of the Thriller that the best thrillers are the equal of the best literary fiction.

There's another problem: general fiction, slimmer by about a third, would still contain lots of romance novels, not to mention the occasional western. Next thing you know, romance fans will demand their own category, and then we at PW will have to worry where to put, say, romantic suspense. (When I was the Reviews editor in charge of SF/Fantasy, I had to keep an eye on the sometimes subtle difference between “paranormal romance,” which didn't belong, and “romantic fantasy,” which did.)

In 2004, thriller writers declared their independence from the Mystery Writers of America by forming their own group: International Thriller Writers Inc. My suggestion? Reunite the two under a common banner in the Reviews section of PW: “Mystery & Thrillers.”