Robert Motherwell (1915-1991), one of the leading American Abstract Expressionist painters, was also a theorist and exponent of the movement. His writing articulated the intent of the New York school --Jackson Pollock, Willem de Kooning, Adolph Gottlieb, Franz Kline, Arshile Gorky, and others--during a period when their work was often reviled for its departure from traditional representation. As founder of the Documents of Modern Art series (later renamed the Documents of Twentieth-Century Art), Motherwell gave modern artists a voice at a time when very few people understood their theories or...