A dapper gentlemen carefully searches the rows of books hoping to find an unread title. "He's probably a mystery reader," whispers Nelson Freck, "because he's an older gent and he's wearing a pin-striped suit . . . If he were younger and was in jeans and a t-shirt, I'd guess he was a sci-fi reader."

The dapper gentleman pays for his books, The Case of the Stuttering Bishop and The Sullen Girl, both Perry Mason novels.

Nelson Freck and Jack Frazier are coowners of Chaos Unlimited, a bookstore at 3809 McKinley Street NW specializing in out-of-print mysteries and science-fiction publications. The store's 15,000 paperbacks, hard covers and a few pulp magazines, most published during the '30s and '40s, are rising in value and sought after by collectors as well as readers, according to the two owners.

Frazier says, "Collecting comic books, antiques and even baseball cards can be expensive -- a Marvel Tales comic book recently sold for $28,000 -- so Nelson and I decided to specialize in used paperbacks." He adds that people are beginning to collect old paperbacks with a keen interest in authors no longer in print, such as mystery writer Carter Dickson and science-fiction novelist Fredric Brown.

One patron is searching for all 600 Dell paperbacks published in sequence during the '40s and early '50s. He's got about 400 after looking for two years. These books featured crossword puzzles or maps on the back cover offering clues to the mysteries or setting the scene for the science-fiction reader.

One customer paid Freck $500 for a 1930 publication entitled Astounding Science Fiction; Freck had paid $225 for the book. Freck also picked up a copy of Dr. Fell -- Detective at a church sale for a nickel and sold it for $20. Freck and Frazier will buy paperbacks, often brought to the store in boxes, and get others from church and yard sales and estate closings.

Many customers collect the old paperbacks because of the cover art some of it pretty racy stuff, especially for its time. Competition among the publishers was intense in the '30s and '40s, and sexy "dust covers" were often provided as an alternative to the more traditional paperback covers. Even some early Agatha Christie novels, not generally known for their torrid love scenes, were advertised with scantily clad women on the cover.

Not all the store's customers are collectors, though: Most are avid readers who have been hooked for years on either sci-fi or mystery novels. For the less specialized books, prices generally range from 75 cents to $3, and the store will buy books back after they've been read.

Bob Snider, a self proclaimed science-fiction nut who works at the store, says that science fiction is growing in prestige. "Many writers began their careers with the pulp magazines -- Tennessee Williams' first published story appeared in Weird Tales," he says.

Freck and Frazier met at American University and discovered a mutual hobby in paperback collecting. Frazier was hooked on mysteries with a special interest in the Chinese mystery novels written by Robert van Gulik. Van Gulik translated a novel from Chinese entitled The Cases of Judge Dee and wrote about 20 subsequent novels using the same characters. Frazier, a law student, was fascinated by the original Chinese mystery novel. Torture was allowed to obtain a confession but if it was discovered that the suspect was innocent, the judge would get the punishment.

Chaos Unlimited is open 11 to 9, seven days a week.