After beginning to answer, Jennings said nervously, "I shouldn't start. You go," throwing the question over to Brewster, which drew laughter from the crowd.
The event, sponsored by Wordsworth Books, was followed by a book signing outside of the lecture hall.
A second book, The Century For Young People, was also available for purchase.
The program began with a speech by Jerome T. Murphy, dean of Harvard's Graduate School of Education.
He lauded the work for its down-to-earth perspective.
"Many histories document the major events, but few show the emotional impact of events through the eyes of those who witnessed it," Murphy said.
"This book portrays the sobering realities of the century while also showing hope for the future. We are all in this together, and together we move forward."