NELL by Nancy Thayer (Morrow: $17.95). Nancy Thayer is noted for what the jacket of this, her third novel, describes as "women's fiction." Like romance novels, women's fiction concerns itself with love; what sets it apart is that the romance is tempered by pragmatism. Love appears but it also regularly disappoints. Thayer's protagonist is Nell, 38, possessed of "a body that would always look good in tight jeans," and also the marks of past pregnancies. She is the divorced mother of two, manager of a chic boutique, and ready for romance. Ex-husband Marlowe St. John is long gone. Will he be succeeded by Steve, the young contractor building the neighbor's pool, who wears "jeans slung low on his hips," as well as "a tool belt slung low on his hips"? Or by Stellios, a Greek manual laborer who is sexy but "rather simple"? Then there is Andy, divorced. He has a daughter away in prep school and a "mammoth, fabulous" house. In a romance novel, Andy would save the day. "Nell" isn't about happy endings, however, but about the stress and misery along the way ("She was so tired," Nell thinks at one point, "of the futile little dance of meeting and mating and running away"). While this novel is relentlessly trite--it is also warmly empathetic. "Nell" is essentially a pep talk to single women, telling them that marriage isn't everything--being alone can also be honorable.