MIRIAM IN HER FORTIES by Alan Lelchuck (Houghton Mifflin: $17.95). In Alan Lelchuck's novel, "Miriam in Her Forties," his sequel to "Miriam at Thirty-four," the heroine steps triumphantly into the clearing. A Boston photographer with two children, Miriam has a bad marriage, a breakdown, therapy--and a rape--behind her. We wait anxiously to see what she does in her new space. But Miriam is such a keen observer of her own life that she becomes curiously abstracted from it. She ruminates about everything--sex, Jews, motherhood, infidelity--one longs for something to happen so that she might react to her world instead of commenting on it. When something really does happen--her rapist returns to haunt her after eight years' absence--it gets buried in the pile of other topical issues on Miriam's mind. What could have been the dramatic thrust of the novel is a series of inconsequential encounters, never satisfactorily resolved. Lelchuck knows the world of New England academia well, and Miriam fits easily into it. But in his effort to have us get under his character's skin, he gives Miriam a voice that is polemical, self-absorbed. She ends up becoming a mouthpiece for the contemporary woman, and the woman herself is accidentally lost along the way.