By Elizabeth Buchan

St. Martin's. 256 pp. $24.95

By Wendy Law-Yone, whose latest novel is "Irrawaddy Tango."

I once asked the prodigious Nora Roberts, best-selling author of more than 126 romance novels, in what ways she thought the genre had changed in recent years. "It's not about an 18-year-old orphan and her love for an emotionally stunted and domineering male anymore," said this publishing phenomenon, whose books in print exceed 42 million copies. "Romance is so much more equal now: no more Cinderella stories. Still, the ingredients of romance are the same: one man, one woman; sexual tension and conflict; emotional commitment; a happy ending." Ask any successful romance writer, and the answer is basically the same: Times change, fashions change, heroines change, but the laws of the genre remain immutable.

Take this novel, a bestseller in the United Kingdom. A mainstream novel on the face of it, with characters and themes at times more suggestive of realism than romance, "Perfect Love" nevertheless remains true to genre, as the title itself insinuates. Adultery against a backdrop of recession, though? Not exactly the stuff of romance, you would think, even given the slow encroachment of issue-oriented topics--cancer, mental illness, single parenting and the like--in some current romance series imprints. "We have no adultery," a Harlequin Books representative confirmed, when I phoned to ask. "Our hero and heroine may be kept apart, but they do have a committed sexual relationship. They never cheat on each other. In our series books we have happy endings. There is no happy ending with adultery."

Oh, no? Just read this book. Suppose adultery were a disease (rather like cancer and mental illness) to be overcome, not punished. Suppose, once overcome, it could even return the victim-heroine to a normal, loving, married life. Suppose Josephine Hart's "Damage" had a happy ending. Suppose all of the above, and what you get is "Perfect Love," a romance novel in the classic tradition.

Prue Valour, the heroine-adulteress, has been happily married for 20 years to a successful, considerably older attorney ("tall, establishment, with the additional advantage of suffering") when she falls in love with Jamie. Jamie is the husband of Prue's stepdaughter Violet; in other words, he is Prue's husband Max's son-in-law. Never mind. The point is: Prue and Jamie have an affair. Prue, age 40, is a dutiful, caring mother and wife. (Beautiful, too, of course.) She has a part-time job at a bookstore, is active in the church and is working on a biography of Joan of Arc.

Violet, the brittle (also beautiful) stepdaughter, now in her late twenties, has hated Prue ever since Prue married Max after his first wife (Violet's mother) was killed in an accident. A tense, ambitious, diet-obsessed publishing executive, Violet is also being driven mad by her colicky baby. Her considerably older husband is an assets manager in merchant banking. Successful and handsome like his father-in-law, he resembles him physically as well. Prue and Max live comfortably in Dainton, a village between Winchester and Salisbury, "girdled by custom . . . riddled with unemployment and weekend cottages . . . marked out seasonally by fetes, Church high days and whether or not its inhabitants wore tweeds underneath their Barbours."

Jamie and Violet and their new baby have just returned from a stint in New York, to settle in north London. The illicit town-and-country affair between Prue and Jamie takes place against the backdrop of Britain's early-'90s recession, we are told, although you'd never know it from the good life these sophisticates lead, with their nannies, Armanis, radicchios, carpaccios, Haagen-Dazs and pinot noir. Like the current-affairs commentaries delivered by Max and Jamie, which show them to be discerning men of the world, the hovering recession serves an ultimately positive purpose: "Recession had changed the face of the village, rather as an unwise love affair had exposed the wrinkles in her marriage."

Unwise. Not disastrous, not catastrophic. (Not even unseemly, for all the urgings, pantings, meltings, judderings, etc., of desire.) Just unwise. Prue betrays her husband and her stepdaughter, but "perhaps all families are busy practicing the point-counterpoint of deceit and revenge. . . . Prue was at it; the next-door neighbour was at it. The nation was at it." There you have it: adultery as epidemic. But treatable, reparable. "Although cracks will be apparent from the inside, possibly from the outside, a smashed vase can be put back together with time and skill."

A fairy tale for the millennium, if ever there was one. And a jolly good read it is, too--as only adultery can be, whether fairy tale or nonfiction. The only snag that keeps this book from being a perfect page-turner is the annoying subtext of the life of Joan of Arc. One of history's biggest bores, Joan rears her righteous head again and again at the most improbable moments as Prue deliberates over life and love. When Max, responding to a question about the biography, snaps, "To tell the truth, I wish I'd never heard of the bloody woman," I almost cheered.