by Ian McEwan. Doubleday/Nan Talese, $19.50. Mr. McEwan has created a semi-fictional memoir based on the lives of his wife’s parents. These people—June and Bernard—were alike in their refusal to compromise their unfortunately incompatible beliefs. Both began as Communists and subsequently left the Party, but he remained a scientific pragmatist while she experienced a religious revelation. They split, and the story of how they reached, and survived, the parting is what Mr. McEwan slowly unravels. The author himself disclaims fixed belief in rationalism, mysticism, or any other set formula. “I am uncertain,” he explains, “whether our civilization at this turn of the millennium is cursed by too much or too little belief, whether people like Bernard and June cause the trouble, or people like me.” He puts an unanswerable question, but explores it ingeniously.