A simple, seamless but at times slow-footed first novel about an adolescent girl's coming of age. Karen Moss, 15 and restless, has been sent by her parents on a summer visit to her mother's sister, Aunt Augusta, at the old family home in New Franklin, Ill., where Augusta, 35 and eccentric by small-town standards, lives alone. While the two of them scrape and paint the big old house, Augusta initiates Karen into family and personal secrets, many of them reassuringly having to do with Augusta's own (normal) sex life at Karen's age--a subject very much on Karen's mind. But soon Augusta's secrets darken: Karen learns that her aunt's distant teen-aged lover, Jerry, is still wooing Augusta, 20 years later; that Augusta has rejected the only man she ever loved, named Allen, who, married, still writes love letters to her from New York; and that her aunt is carrying on an unhappy, local adulterous affair that Karen's visit was intended to help end. In the meantime, Karen meets a strange, charming boy who falls in love with her, and the parallels between her choices and those of Aunt Augusta deepen and grow uncomfortable. When Karen's parents cut her summer short by calling her home to talk about their marital problems, Karen is both wiser and unaccountably relieved--having learned that decisions do not always adhere to her childhood sense of right and wrong. Pei's sympathetic identification with Karen is tonally faultless, though sometimes superficial. Still, Family Resemblances should have been a YA book: it is flat, familiar and simple in a way that isn't likely to satisfy adults.