Regretful - that's the right definition for what I felt about this book!
As he had already written on the other event that shocked America ("Libra" was about the Kennedy assassination), and as he lives and was brought up in New York, it was only a matter of time for DeLillo to write about 9/11. This book is not a masterpiece, yet it shows how DeLillo's skills have constantly increased through the years. The idea of the performance artist known as "The Falling Man" is one of his best, and his description of the life of one of the survived and his wife is haunting. They are only a couple of people in an enormous crowd, and their actions and problems seem shallow and empty and banal: yet, they could not be more significant. This books show what it must really mean to live through those terrible days, the absurd quality that life had took on itself.
I only regret the fact that DeLillo felt the need to tell about the actual facts that brought about 9/11. The brief chapters on the terrorists are the least realistic of the novel, looking like ordinary docu-fiction. It would have been far better if he had left the Towers' ruins on the background: they would have haunted the reader even more.