A sensitive exploration of what it means to be alone; Perfect Happiness, by Penelope Lively. Garden City, N.Y.: The Dial Press. 240 pp.$14.95.

Merle Rubin reviews books regularly for the Monitor. As the novels of a former time often ended with a marriage, so do many of today's novels begin with a marriage's end. The flowering of consciousness-raising groups and a rising divorce rate may account for some part of this change. But a high proportion of contemporary novelists seem concerned with the more timeless themes of widowhood and loss that is unchosen.

In place of young women like Jane Eyre and Elizabeth Bennet poised on the brink of future happiness, we find ourselves constantly reading about older women, like the widowed heroine of Penelope Lively's novel, whose happiness has been in the past. ''Perfect happiness, past perfect, pluperfect,'' muses Frances Brooklyn, a reflective and intelligent woman whose sense of permanence and very identity are threatened by the loss of her husband, Steven, a well-known BBC commentator.

Unlike some novels that focus on the erosion of social identity and status, which widowhood may bring, ''Perfect Happiness'' is more concerned with the psychological - dare one say metaphysical - aspects of suddenly discovering what it means to be alone. Frances not only misses Steven and the happiness they shared, she is also tormented by the thought that, if she were to forget anything, both Steven and those past moments may as well never have existed at all.

While Frances struggles with her grief and fear, Zoe, her close friend and sister-in-law, confronts the specter of impermanence in slightly different form: Although she has deliberately and, she thinks, wisely chosen to remain single to pursue her career as a journalist, she must come to terms with her regrets about not forming a profound relationship with a man.

Tabitha, the Brooklyns' adopted daughter, is experiencing her first taste of love and loss, while Morris, a middle-aged musicologist, feels that he is about to rediscover ''perfect happiness'' through Frances.

Lively, whose latest book, ''According to Mark,'' (not yet available in the United States) has been nominated for the prestigious Booker Prize for fiction in the United Kingdom, captures in ''Perfect Happiness'' the precise heft of her characters' moods and emotions. Her novel is distinguished among others of similar theme and subject matter by its artistic restraint and its sensitivity to language as well as mood. In writing what is manifestly a novel of sensibility, Lively manages to focus on subjective states of being without abandoning her own objectivity, so that she is able to evoke her characters' complex, sometimes inchoate, feelings with precision and subtlety.30