Once upon a time, the category of nonfiction referred to ghost-written memoirs and 700-page discourses on the indigenous tribes of Quito, Ecuador. It was the section of the bookstore that everyone passed by in order to grab the latest novels by Tom Clancy, John Grisham or Danielle Steele.

But lately, there's been a slump in quality hardcover fiction (The Bridges of Madison County, anyone?). Nonfiction books have taken advantage of that slump, using it as an opportunity to rocket up the bestseller lists.

A brief look at last year's bestselling books shows that nonfiction might soon be on top. William Bennet's The Book of Virtues, which spent 50 weeks on the New York Times Bestseller List, is a prime example.

The success of nonfiction is due mostly to an overall shift in attitude and subject matter, a shift that has been noticed by book sellers, publishers, critics and authors.

"Nonfiction is very interesting," said Mark Cain, sales associate at Encore Books at Hills Plaza. Lately, the subject matter in nonfiction has become a lot more exciting than usual. "It's getting more like fiction."

It is true that nonfiction has become "fictionalized." The Hot Zone by Richard Preston, and John Berendt's Midnight in the Garden of Good and Evil are based on true stories but are written in a breezy, entertaining style that makes readers wonder what is fiction and what is not.

"I like them," Cain said, adding that the books seem to bring many more readers to them. Sales of both books have been very good, he said.

The success of those two books might lead to more stories written in the same style.

"I wouldn't be surprised if there were more of them," said Robert Shepherd, owner of Little Professor Book Center, 1647 N. Atherton St., referring to the more exciting style that many new nonfiction books have adopted.

Publishers agree that the change has resulted in boosted sales and a renewed respect for the form of nonfiction.

"Nonfiction outruns fiction on the bestseller lists," said Kate Fischer, spokesperson for Simon & Schuster. "(It) has done very well for us."

Fischer added that various publishing houses take different approaches to nonfiction. "It depends on what you want your company to do."

Simon & Schuster's bestsellers for the year included Gail Sheehy's The Silent Passage, the immensely controversial The Bell Curve and Bennet's Book of Virtues.

Nonfiction has had a new surge of respect from the critics as well. In its year-end issue, Entertainment Weekly magazine's book reviewers listed eight nonfiction titles, including The Hot Zone and Midnight in the Garden of Good and Evil, on its annual top-ten list.

To make things even more confusing, authors of fiction have started incorporating real-life characters in their work. Caleb Carr's The Alienist features Theodore Roosevelt when he was police commissioner of New York City. The corrupt Boss Tweed plays an important part in E.L. Doctorow's The Waterworks.

Other fiction bestsellers, like James Redfield's The Celestine Prophecy and the notorious Iron John by Robert Bly, are philosophical musings on enlightenment thinly veiled as a novel.

It remains to be seen whether this nonfiction boom will continue and if nonfiction sales will keep on rising. But a shift in attitude and subject matter has injected a new zest into a tired genre, a fact that may change the way we think of nonfiction forever.