First mystery fan Bill Clinton ducked into Philadelphia's Wyndham Franklin Plaza Hotel, site of Bouchercon 29, to rest up for a Democratic fund-raiser October 2. In keeping with the convention's theme, the president managed to remain mysteriously invisible throughout his one-hour stay.

His Secret Service agents made a much better show. Two by two the agents trooped across the glass and marble-walled lobby. The heads of each pair turned in unison as their steely gazes scanned the chatting women in discreet pantsuits and sensible shoes.

Every so often an impossibly square jaw would clench as a high-pitched discussion of poison or explosives carried across the room. For an instant the agent would hesitate, as if he'd forgotten the next line in the script he was feeding through the slender mike on his headset. Then he'd realize no one was storming the column and resume his endless report.

Mystery writer Kate Gallison stared grimly at the robotic procession of short hair, white shirt collars and stolid blue suits. "Look at the way they move," she said at last. "They give me the creeps."

But to a certain extent "the creeps" are why people go to Bouchercon.

Bouchercon (pronounced BOW-cher-con, in honor of Anthony Boucher, the late mystery-loving critic of The New York Times) is an annual gathering of about 2,000 mystery writers, publishers, retailers and fans. Unlike some cons, Bouchercon travels from place to place.

From Thursday to Sunday, October 1-4, the convention made its home in Philadelphia, one-time residence of Edgar Allen Poe, the founder of the modern mystery. Organizers made the most of the mysterious side of the city's history, offering tours of Poe's house, Laurel Hill Cemetery, and the evocative, castle-like ruins of Eastern State Penitentiary.

Philadelphia forensic sculptor Frank Bender displayed works that helped police identify the missing and track killers. Bender often works with the Philadelphia-based Vidocq Society, a group of law enforcement officers, forensic specialists and private investigators who donate their time and talents to help police in solving "unsolvable" crimes.

Together with Bender, the society presented a panel on Philadelphia's most famous murder case, that of "the boy in the box." The Vidocq's Society's efforts on that case also was featured October 3 on the prime-time television show, America's Most Wanted.

On the lighter side, Gentry, the city fire department's arson-sniffing dog, scored a big hit with his own panel on Friday afternoon. British mystery author Simon Brett offered a staged reading of his play Silhouette. Con-goers could also attend a mystery dinner, "Murder by the Book," and a new musical based on the Parker Brothers board game Clue.

Like most mystery conventions, Bouchercon 29 focused on providing writers and would-be writers the tools to create and market murder. Bounty hunters, federal agents, policemen and private investigators provided insider details on their respective crafts.

Panels addressed subjects like "the creative use of fear," "the ethics of make believe," plotting, setting and dialogue. Even star guests like Stephen J. Cannell and Stuart Kaminsky offered tips on how to make ensure every story has a solid "second act." Pros and fans alike took notes.

This attention to the business of producing books meant the sense of play that links all genre conventions took a decidedly cerebral turn. The more daring the plot, the more outrageous the device, the better, as long as it remained only talk or words on a screen. Acting-out -- even the necessary posturing of the President's protectors -- was a different matter.

Women in silver button earrings and men in rumpled flannel coats peered in disbelief at the half dozen reed-like young publicists and assistant editors wafting from room to room. The unrelieved black of the interlopers' clothes, the dyed-dark hair, raisin-colored lips and exaggerated combat boots appeared more exotic than khaki pants at a science fiction convention.

Someone urged this reporter to pay special attention to the back row in the dealers' room. "You won't believe the things they're selling in the corner. And people are buying it! There's got to be a story there."

The dealer in the corner turned out to be the same vintage clothing shop that does land office business at Malice Domestic, the cozy mystery convention held each May in Washington, D. C. At Malice, the shoppers line up three-deep. At Bouchercon, there was always plenty of room in front of the large jewelry case. Bouchercon attendees came to decorate the inside of their heads, not play dress-up.

People ebbed and flowed around the other merchants of mystery-related gifts and souvenirs. Books were con-goers' drug of choice. The roughly 90 booksellers and small publishers occupying the front rows of the dealers' room seemed to spend the entire con restocking and rearranging their shelves to cover bare spots. Many sold enough to leave early and beat the traffic home.

The dealers' efficiency mirrored con operations as a whole. Panels ran an hour each with half-hour breaks between. Everything started on time. Every room boasted enough chairs, water-coolers and cups.

But cerebral and efficient though Bouchercon was, nothing was cold except the water and the weather outside. Organizers and attendees saw the event as a combination reunion and christening, designed to welcome new members into the mystery "family." Awards presentations celebrated the mystery family of today, while "veterans" helped build the mystery community of tomorrow.

Industry insiders forged connections between the new and the old. Friday night, for example, writer Les Roberts played and sang for three hours on the Yamaha grand piano next to the lobby bar. St. Martin's Press editor and Bouchercon honoree Ruth Cavin sang with him and encouraged friends and fans alike to join in.

At the "first timers' forum" for newly published and about to be published mystery writers Sunday morning, "old hand" Margaret Maron said: "I remember the first time I came to Bouchercon. Everybody seemed to know everybody else. They were getting together and talking, and having a great time. And I felt a little left out.

"If you feel that way, look around you. See who looks a little left out and start talking to them," Maron continued. "That's the way you make friends with people like you. And it's important to make these friends and connections, because this is your tribe."

Jean Marie Ward