Listen to the audiobook if possible. Multiple readers give the whole experience a haunting, more human feel.
You'll notice right away that the book skips from interviewee to interviewee, sometimes after only the barest hint of a description. At first this might seem jarring because, well, you want to hear the full account. Have patience, dear reader/listener. Telling the story of 9/11 works better this way. It brings everyone's story forward together, and thus maintains the correct emotional tone from start to finish. In fact, it worked too well for me. I had to stop listening at times because the immersion was so overwhelming.
The Only Plane in the Sky will certainly reveal new insights about that horrific day. And given the passage of time, these insights will color our "Never Forget" memories of 9/11. There's a comment near the end by one of the interviewees, which I'll paraphrase, "Not a day goes by where I don't think about 9/11." It's like that in small ways for the rest of us. The closer we were to the event, either physically or via memories or through family and friends, the more 9/11 takes up space in our daily/weekly/monthly thoughts. And it will be so for a long time to come.