Multiple times. It's the best post-apocalyptic novel ever written, bar none.
It's a shame that George Stewart has largely been forgotten. He wrote Earth Abides on a lark; most of his stuff was essays. Earth Abides started out as essays, in fact; he was writing a bunch of essays that were essentially The World Without Us and tying them together with narrative about the experiences of a lone survivor. About halfway through he realized that the interstitials were much more engaging than the essays so he swapped their prominence.