On the morning of September 11, 2001, Paul McCartney was in New York City on an airport runway waiting to fly to Britain. As he absorbed the news of the unfolding tragedy, he wondered, “What can I do?” The answer, of course, lay in music. 　　McCartney reached out to master documentarian and long-time friend Albert Maysles, inviting Maysles to document his personal experiences on 1...