First off, let's talk about what ending counts as the ending: does that mean the end of Chapter 21? The end of the epilogue? The end of the afterwards? The end of the bibliography? The end of life as we know it?
Okay, it's probably not that last one. Still, the last numbered chapter ends with Marcus holding Darryl's hand in the hospital (21.86). Marcus almost says, "I love you" but he holds back. If the book stopped here, we'd probably assume that everything turns out a-okay for Marcus, Darryl, even the older guy in the next bed over.
But then something tricky happens—there's an epilogue. This section ends with Marcus and Ange kissing, and then going to get a burrito (Epilogue.61). Are we supposed to think Darryl and Ange are equally important to Marcus's life? Or is this more a soft cliffhanger preparing us for the sequel that may or not come?
Then there are the two afterwords—they both end telling readers to go out and do something. The end of Doctorow's own Bibliography section lists books that inspired him (perfect for doing comparison work) and thanking the people who inspired him.
So here's our question: does Marcus end the story? Does Doctorow? What changes depending where you mark the ending? What are those other sections even doing there?