AMONG BIRCHES. By Rebecca Hill. Morrow. 323 pp. $17.95.

Rebecca Hill's second novel -- her first was the highly accomplished "Blue Rise" -- is a report on a skirmish in the war between the sexes. Its principal characters, Aspera and Will Hancock, are turning 40 and heading for trouble; their marriage, happy for many years, is increasingly tense and unfulfilling, while within the small circle of their friends separations, divorces and affairs raise the prospect of tempting alternatives. Their marriage has lasted for two decades; the question now is whether it can survive the arrival of both partners at the middle of their lives.

Yes, it is a question that has been raised before. What has been called the "divorce novel" -- whether the marriage actually ends in divorce is immaterial -- seems now almost a rite of literary passage for American novelists. Given that divorce is a fact of middle-class American life this is not surprising, but the genre is beginning to show signs of wear. Divorce no doubt will be with us forevermore in fiction, but as the central preoccupation of novel upon novel it has gradually lost its former urgency.

In "Among Birches" Hill probably breathes as much life into the familiar story as can now be hoped for, but it remains that the story is familiar and that the characters, most of them retreads from "The Big Chill," are even more so. Hill is an intelligent writer with a firm grasp on the minutiae, both actual and psychological, of daily life, so there is much in the novel with which the reader can connect; yet a thrice-told tale it is, and with structural difficulties that further undermine it.

The one character who achieves real identity and interest is Aspera Hancock, which is fortunate inasmuch as she is the center around whom the other characters revolve. She is a well-educated person who had been training to become a librarian when Will inherited his family's small business -- a lumberyard in a town outside Minneapolis -- and decided to operate it. She took over its business management and nursed it back to health. Now, on her 40th birthday, she wants to leave the business and resume her own interrupted career. "I want to leave the store," she tells Will. "I'm not trying to leave you." But this he cannot and will not understand, and thus the marriage gradually descends into "deadness."

The difficulties within the Hancock household are compounded by the constant presence in their lives of their friends -- "her dearest friends, the people who cared for her" -- whose own lives have grown ever more complicated and who draw the Hancocks into a "web of treacheries." The friendships do not disintegrate, yet there is trouble: "Was it possible for all one's friends to go dangerous at the same time? Possible that misunderstandings operated on an orbital cycle like Halley's comet, coming around in one big ball every so often, and this was it?"

One of these friends, a 42-year-old widow, brings a lover into the group: 26-year-old Kevin Stowe. Will and Aspera ridicule him as Baby Kevin, but he turns out to be a tougher customer than he had at first seemed: a compulsive womanizer who soon enough lures Aspera into his bed. At the outset she tells him that "this is going to mean a lot more to me than it does to you," and of course she is right. The affair is a thrill a minute -- "Thrills and chills and no piper to pay" -- but it also forces Aspera to ask hard, serious questions about the seriousness of marriage and her own commitment to it; by the time she is ready to face them head-on, the further question is whether her marriage to Will can be salvaged.

Aspera is no active feminist -- she tends to wince when her friend Bunny veers off into feminist rhetoric -- but she is up against a dilemma that is familiar to countless thoughtful, educated women whose lives have been touched by the new liberation. On the one hand she wants to be herself, on the other she wants to be her husband's wife; this isn't trying to "have it all," but to reconcile the conflicting demands of self and family. She wants both love and liberty; whether she is willing to make the necessary compromises is what she -- and, from his side of things, Will -- must decide.

Though Aspera is sufficiently bright and appealing to make the reader care about what happens to her, her story is diminished both by its familiarity and by an overabundance of heavy, protracted talk in which deep issues are belabored to a fare-thee-well. "Among Birches" has a serious show-and-tell problem; Hill needs to trust her readers' intelligence more, to let her themes evolve naturally rather than in long conversations that are considerably less interesting than the participants think they are. She is a skillful and compassionate writer; it can only be hoped that "Among Birches" gets the hot air out of her system.