THE STORY THIS FIRST novel has to tell has beentold before, with minor variations. A woman married to a much older man finds herself falling in love with his handsome young son by an earlier marriage--in this case, a boy just three years her junior. She grows obsessed; she hides her secret less and less well; and eventually, she comes to a tragic end.

It's been told before but never, I believe, quite so hauntingly, in such exact and evocative language, with such a delicate sense of pace. Not a moment in Pretty Redwing is rushed. Events unfold almost imperceptibly; they take on color while we watch. You can read this book in the dead of winter and develop a thirst for iced lemonade, so pervasive is the atmosphere of a warm, slow, sleepy Southern town in the late 1930s.

The narrator is the woman's young daughter, a child who is at once extraordinarily fearful and oddly tough. It's the perfect combination. Because her fears lead her to follow her mother's every move, she can observe in uncommon detail without striking the reader as precocious. And because she is so unflinching, so stony of gaze, she can tell us what she observed with an accountant's precision.

She can tell us of her mother's unvarying routine-- her visits to Belk's pattern counter, her mornings spent sewing, her afternoons spent languorously bathing and dressing, her conversations with her women friends:

"All their conversations were like so much formula. Like prewritten dialogue from a play. It was as though each woman had chosen a character and sketched it in. When the other women spotted it, recognized it, they gave her her designation and thereafter all conversation was easily fitted to the chosen part. Winnie was oppressed. So when the topic of subjugation came up all looks were directed to Winnie, she was the authority. Leita was a flirt. Trudy was the rich one. My mother, Flora, hated to shop for food and plan menus. They each knew their own part and never tried to steal each other's lines or characterizations."

By the time we're familiar with Flora's days--and with her houseful of furniture selected by the first Mrs. Simpson, and with her elderly, loving but often absent husband and her self-contained stepson--we've come to sense her secret. It doesn't have to be spelled out; it grows on us. There's a gradual realization of Flora's discontent, and of the stepson's cool appeal. The child narrator, like some anxious little seismograph, positively trembles with dread; she half understands and half does not. She dreams of an intruder seeking entry into her house. She dreams that if she avoids looking at the intruder, he'll leave. She believes that something is hovering just beyond her reach, and she calls this something Now You Know.

But just because it doesn't need spelling out doesn't mean it's not spelled out, and that's the one minor flaw of Pretty Redwing. Like many another first novelist, Helen Henslee underestimates her readers' perceptiveness. This leads her to make two clear mistakes in the area of viewpoint. First, she slips in a few observations unquestionably beyond the ability of any child--even one so watchful, even one looking back, as this one is, from a much later time. The child describes, for instance, the private, silent memories passing through her mother's mind as she lies semiconscious on a sickbed. And she states, with an omniscience totally out of keeping with the rest of the book, that her father "didn't know what it was Flora wanted, but he knew it was something he had been unable to give her. He felt her dissatisfaction even through their lovemaking, even though he knew he satisfied her physically."

Second, there are several short sections in which the observer is suddenly (and jarringly) Flora. It's easy to see the logic behind this. These sections are the ones dealing explicitly with sex. They describe Flora's marriage, her sexual fantasies, her obsession with her stepson, and a sleazy, desperate episode in a motel with a chance male acquaintance. Obviously, no child could give us the information. But why do we need it? Since the child has already told us, for example, that Flora married a man the same age to the day as Flora's own father, is it necessary to know too that Flora's fantasies involve violation by old men?

Luckily, these sections form only a small part of the novel. Elsewhere, evidence a child could easily accumulate--Flora's restlessness and sidelong glances, her irritability, her mood swings--informs us deftly and convincingly. Just watch Flora's daughter explain why she mentally links her mother and Randolf, the stepson, although she has no conscious notion of the significance of that link. It's a sample of childish reasoning at its purest, entirely plausible:

"I thought about it in terms of arithmetic and trying to form the more equal and therefore more equanimous pair. If I subtracted my mother's age from my father's there was a big number left over. If I subtracted Randolf's age from Flora's, however, only a little number remained. That made Randolf and Flora more equal. That gave indication of who belonged with whom, in a way, I thought."

You might suppose, considering the somberness of the plot, that this is a grim sort of book. It's not, oddly enough. Maybe that's because it's so rich in atmosphere, so lovingly complete in its treatment of each character, that you feel an unexpected pleasure even as you mourn what happens. Flora's women friends--flashy, common, a bit off-color--have surprising power to touch us when we watch them clowning in an auto showroom, each behind the wheel of a different car and pantomiming a wild, careening joyride. Flora's daughter bringssback to us all the gritty details of childhood--the Sunday school leaflets that smell as if "they had been brewed rather than printed," and the perforated metal curlers like a "group of baby Louisiana crocodiles," and the salad of half a canned pear filled with mayonnaise on a bed of iceberg lettuce. People so solidly placed within their settings, so very much themselves, can only delight us-- and in many ways Flora is the most delightful of all, despite the unhappiness she causes.

At one point, Flora spins a gyroscope and lets it balance on the edge of a glass. "Look at that!" she tells her stepson. "Perfect balance. It doesn't lean too far one way and it doesn't lean too far the other. I admire it. It's daring. It's daring and it's brave." When she says that, you have to admire Flora herslef. She's a fragile, teetering, heartbreaking woman with her own misguided kind of courage, and her story as Helen Henslee tells it is a little miracle of grace and skill.