Composing can be a lonely job. Hour after hour, the composer sits at a piano, trying to bring out the music in his head. But for Aaron Copland, composition was about the people. Working with people, learning from them, befriending them, organizing and mentoring them. His enthusiasm for combining music with humanity assured his place in music history almost as much as his compositions. "As organizer, teacher, propagandist, critic, lecturer and expositor, he has been by far the most voluble, articulate and respected American musician of his time," wrote Harold Schonberg in The New York Times in 1970. His ballets...