LEAVING HOME: A Memoir by Art Buchwald (Fawcett: $12.; 254 pp., illustrated). Fans of Buchwald's political satire will be surprised by his poignant autobiography. The son of a mentally ill mother and a father ruined by the Depression, Buchwald spent his childhood in orphanages and foster homes. An unhappy, confused boy, he developed his sense of humor into a shield and a way of avenging the many hurts he suffered.