Detente in the family room; Stepping by Nancy Thayer. New York: Doubleday. $10.95.

Every so often a novel appears which deals with contemporary life in such a frank, uncompromising way that one might fear its edging over into sentimentality or melodrama. "Stepping" never does. It's the story of a young woman eager to begin married life with the man she loves and respects -- only to discover the union comes with two angelfaced enemies, his two young daughters by his first marriage.

Told with directness and humor, Zelda Campbell's experiences with "stepping," or stepparenting, two unwilling subjects might offer real-life stepparents both insight and refreshment.

Not that Zelda and Charlie Campbell do everything right, or that it's perfect in the end. Twenty-one and a student when she married her 36-year-old professor/historian, Zelda finds life idyllic for the first months, just the two of them in a round of academic interests, cultural events, and weekends at their small farm in the country.

Nancy Thayer's lucid, uncluttered prose anchors their relationship to real life in subtle ways -- physical details of shared experience which also suggest the underlying values and ideals held in common: ". . . the farmland was even more ours [than their small house at the university], or perhaps, since land never belongs to any one person, but remains solidly, placidly, firmly its own, I should say that the land was even more dear to us. It was rocky, craggy, rough-cut land, the kind that causes you to stumble when you walk . . . ."

And Thayer is careful to sketch in the emotional landscape as well -- the young couple in a contented newly wed harmony of work and play that seem the same thing.

The advent of Caroline and Cathy, along with five-page letters of instruction for their care, backed up by frequent hysterical phone calls from their mother, brings the honeymoon idyll to an abrupt halt. Slowly, however, father, stepmother, and children do learn to accommodate each other -- only to have the process interrupted by the scheduled return of the latter to their mother in the autumn.

Their erratic pattern of conflict, understanding, and growth over the next decade, through the birth of Zelda and Charlie's two children and their own maturing relationship, makes a story both fresh and personal -- and, though the contemporary reasons for this are lamentable, widely appealing. Perhaps the insights shared here into conflicting expectations and needs, confused emotions on all sides, moments of breakthrough and trust can help others cope better with similar situations. Even better, they might encourage couples of any age to bring deeper thoughtfulness and commitment to their union and to the beginning of a family.