IT'S TOO BAD Judith Guest used the title Ordinary People, because it would be perfect for Nancy Thayer's latest novel. The characters in Nell are not only ordinary, but they lead astoundingly uneventful lives. They go to see Flashdance, cope with flea-ridden house pets, and get dressed for work. Because Thayer is such a keen observer of the commonplace she can occasionally make the mundane appear special and original: "When she was especially tired or anxious, she looked through the glossy, optimistic pages of mail-order catalogues. She was oddly reassured by the knowledge that this was a world in which there were people who spent their lives inventing dog jewelry or writing such copy as "Nouveau is better than not riche at all.'"

Nell, the protagonist of Nancy Thayer's fourth novel, is divorced, the mother of two children. She sells women's clothes in a boutique by day and by night curls up in bed and reads, or, if she's seeing a man, curls up in bed with him. Hers is not a life on the fast track.

But there is a problem with all this unrelieved ordinariness; as in real life, it is often boring. The author goes on at such length about the most prosaic details of Nell's daily routine that the reader ultimately yearns to escape the way Nell does, by curling up with a good book.

This is not to say that ordinary people are not fitting subjects for fiction. Novels from Madame Bovary to Final Payments have proved otherwise. But the occasional well- wrought sentence is not enough. The reader must care about what happens to a protagonist. Even though a character may appear ordinary, he or she must be finely-drawn enough to be unique, to mean something.

Relationships, too, need texture, and there is little of that in Nell. When Nell's employers ask her to manage a boutique on Nantucket Island for the summer, she meets Andy, an attractive man who looks like a cross between Anthony Perkins and Abraham Lincoln. The reader anticipates something big and, indeed, is told that Nell does fall in love. But it is never clear why, since right from the beginning Andy seems a churlish fellow. He asks about her job: "' . . . it's not very important work, do you think? Not really a challenge?'

'You're a real charmer', Nell thought . . ." He is never developed any further, so despite his hazel eyes, dark brown lashes, and grand Victorian house, Andy's appeal remains a mystery. And since this is the most emotionally charged relationship in a novel where so little else takes place, the book never comes alive.

There are other characters besides Nell and Andy, of course: Nell's children, a step- daughter by Nell's ex-husband's first marriage, a rich, rather vacuous friend, several blue-collar lovers. The problem isn't that these characters are not well-described; there is no doubt what they look like. The problem is that they are not in the least engaging. The reader has little sense of what makes them tick, of why they are drawn to Nell, or (except for her children) why Nell is drawn to them.

There is little narrative thrust in the novel to propel the reader onward. The trip to Nantucket and the return to suburban Boston provide the only movement. As for action, the only inherently dramatic event is the near-drowning of Nell's son. But such an event is intrinsically compelling; everyone dreads the death of a child. Jeremy, the character, evokes no real concern because he himself is not real. He is simply words on paper.

But Nell isn't a book to be tossed aside; it's not terribly good, but it's not bad either. Most of the time reading it is like half-listening to the murmurings of an old friend -- comfortable, soothing even, but not at all interesting:

"When she was younger Nell had worn her hair in odd, extravagant ways: pulled up to the side in a sprout of ponytail, or braided when wet so that it frizzled out softly all around her face in the style of a pre-Raphaelite heroine's. But now she had laugh lines around her mouth and eyes and, when she was tired or worried, little bluish pouches under her eyes; she could seldom get away with flamboyant hairstyles now. When working, she wore her hair pulled back in a chignon or she let it fall down and loose, held off her face demurely with a headband or clasp."

In her other novels, Nancy Thayer showed an admirable ability to select a face in a crowd and draw it so well that it seemed three-dimensional, alive. In Nell, she does not succeed as she has in the past. The face remains a mere two-dimensional sketch.