[Up in Seth's Room] is strictly for teenagers, although the under-12 Judy Blume crowd will probably sniff it out.
The questions we follow relentlessly from beginning to end are the perennial ones of adolescence: Will she or won't she? And what's it like? Fifteen-year-old Finn says she won't. It's too soon, she's too young, and she's not ready…. Seth, who is 19 (old enough and definitely ready) is the brother of the man that Finn's older sister is living with. "Living in sin," as far as the parents are concerned.
Well, to make a long story short, everyone should be pleased with the outcome. Finn sticks to her guns, although the fact that she "doesn't" is hardly more than a technicality. There are enough explicit scenes to give young readers who don't know a good idea of "what it's like."
Although Seth finally lets up on the pressure, he almost immediately moves away. A perfect ending for first love: no more temptation, parents relieved, girl's principles intact, and a goodbye forever scene that justifies a nice cry. If the characters are less fully developed than those Mrs. Mazer has given us before, young readers probably won't notice. They'll be too busy turning the pages.
Jean Fritz, in her review of "Up in Seth's Room," in The New York Times Book Review (© 1980 by The New York Times Company; reprinted by permission), January 20, 1980, p. 30.