There's not much to tell about Tina C., a high-pitched twelve-year-old whose girls' club exchanges beauty tips and who overreacts with prolonged fury and embarrassment when dreamboat Tiger Rawson calls her ""Tina, the Teen-age Twitch"" because of the way her nose twitches when she's nervous. Elsewhere she's concerned lest her father marry his phony tennis partner Rosebud and uneasy about her mother's new live-in friend Peter--who is pleasant and all but, strangely, does the shopping and cooking while mother works. But a week in the city with mother and Peter and, most important, with a new friend, Johann, from Holland, convinces suburban Tina that Peter's okay and that being yourself is more important than looks. Unfortunately her conversion as projected here is as superficial as her previous assumptions; we never see her actually experiencing either a slow or a sudden understanding that readers can share. And as for the trendy parental rematches, liberated mothers have become so obligatory in juvenile fiction that Perl's halfway endorsement (Peter and Mom turn out to be married, and what's more he teaches cooking and plans his own gourmet school, proving that he's properly achievement-oriented after all) comes through as a cop-out.