Don DeLillo's Falling Man.
It's about as good as you expect a DeLillo book about September 11 to be. Which is to say that it's good, and if you like DeLillo's writing, you'll probably like the book, but also that if you've read other books by him (especially Libra), little in it will surprise you.
(And as far as books that attempt to grapple with moments of individual historical tragedy, I found Libra to be much more thought-provoking, especially in its implicit claim that we make events like this happen. Not that they're our fault necessarily, but that we create them. We will them).
And this is DeLillo who, after all, in one of the best moments of White Noise, claimed that "All plots tend to move deathward."
But this is also DeLillo who so perfectly captured the entire second half of the 20th century in Underworld that Falling Man, in its introduction to the 21st century, is bound to feel a little anti-climactic.