This week, two stories from the World Trade Center as it once was: Kurt Andersen talks with Philippe Petit about his daring tightrope walk between the towers in 1974, and Stephen Vitiello composes music using the sounds of the building. Plus, the novelist Jonathan Franzen — his epic best-seller about an American family, Freedom, is out in paperback this month. And we consider Georgia O’Keeffe and her paintings of animal skulls as part of our American Icons series.