Two smart single Manhattanites, having dinner not long ago at the trendy SoHo restaurant Balthazar, were discussing a series of books they'd like to do -- books that would be savvy city guides for people like themselves, stylish young career women. On the way home, one of them dropped into a popular bar, and found herself explaining to a guy she met there that she and her friend were looking for an agent. He turned out to be William Clark at the neighborhood Vines Agency, and in almost no time, he had found a publisher for Jayne Young, a record company exec, and Sheridan Becker, a party planner, for the series they call What's a Girl to Do? After taking the two women around to a handful of publishers, he made the sale to Kelley Ragland at St. Martin's Press, an editor who d s mostly fiction, including mysteries, but responded, as he had hoped she would, to the style and tone of the authors and their lively proposal, which has the feel of the Zagat guides and the contemporary visual look of Britain's Wallpaper. "They're great fun, smart and appealing," said Ragland, "and if this takes off, as we hope it will, as a sort of name-brand franchise, it could embrace more than just travel guides." Already there is talk of doing "What's a Girl" titles on such subjects as getting married and bringing up kids. But meanwhile, the first two city titles, tentatively scheduled for spring 2000, will focus on those singles meccas, New York and San Francisco, to be followed by other American cities, and then some European ones. Young and Becker will supply text and artwork, and SMP will put it all together.