ANOTHER MARVELOUS THING. By Laurie Colwin. Knopf. 130 pp. $13.95.

SETTLING DOWN with a new book by Laurie Colwin is a particularly civilized pleasure, like arriving at a dinner party you have long looked forward to. Your hostess has seen to everything, and yet the arrangements seem to have cost her no trouble at all. You instantly take to the other guests, who are not only warm, charming and intelligent but make you feel that you are too. You dine wonderfully, basking in the hospitable glow; you have made new friends by the time you leave, and your thanks at the door are entirely sincere.

And yet, if you are a boor like me, you will begin to wonder before you reach home why no one argued, or drank too much, or gossipped, or told a vile joke, or mentioned money or politics or some other unseemly topic. Who are these perfect people, anyway?

In Another Marvelous Thing, Colwin's eighth book of fiction, they are Francis and Josephine, known to each other as Frank and Billy. And they aren't absolutely perfect -- they are conducting an illicit affair.

Francis is tall, urbane, and a good deal older than Billy, who hasn't been married long. They are both economists -- Billy is still writing her dissertation -- and both are part of a "dashing couple." Francis' wife is a decorator, although her taste is anathema to Billy; in Francis' done-up house, she feels "imprisoned in a tea cozy." Billy is deliberately plain in her emotions as well as her tastes, and "the fact of Francis was the most exotic thing that had ever happened to her."

They set each other off nicely. Francis is garrulous, nosy, and hopelessly euphemistic; Billy is laconic, secretive and unflinchingly direct. He is a showy dresser who likes elegant post-coital snacks; she shuffles around the house in broke-back shoes repaired with electrical tape and serves her lover hard biscuits.

What they share is a fascination for the other and an agreement that neither marriage shall be disturbed. In fact, Billy tells Francis in her straightforward way that "You're my pet . . . You're my child substitute until I can make up my mind about having a child." Frank cringes, as he always does when Billy states a truth; but he accepts this condition just as he accepts her other terms, "limited doting, restricted thrall, and situational adoration."

And so, deftly, Colwin has detached this love affair from any context, isolating both the lovers and her theme. In this collection of eight connected stories, she reflects often on the strange privacy of lovers in the world of their own creating. Frank and Billy become conspirators, collaborators, co-rulers of their own small kingdom; their love affair is often compared to a "work of art"; it is "this edifice, this monument, this civilization known only to and constructed by the two of (them)."

Frank and Billy have their moments, Colwin captures them beautifully. But they both shy away from scenes and messy emotions; they stick to the terms of their agreement, and the most difficult questions remain "unaskable." Their frequent couplings are described not only with dispatch but with clich,es, as are the times of emotional distress: "That night, like many other nights, Francis felt he was wrestling with demons."

All of which is to complain, mildly, that nothing much seems to be at stake in this book. It draws its energy from a sexual passion; there's supposed to be a fire -- but there's not a trace of smoke.

Colwin, of course, is a writer who places understanding above feeling. Hers is a prose style that aims at, and achieves, clarity. Wisely, perhaps, she has steered clear of the more volatile elements in this book -- but in the last two stories of the collection, she shows her command of emotion. Here, with no loss of clarity, with conviction and feeling, Colwin delivers the marvelous thing promised in her title.