Friend used to work for Life magazine, and was, at the time this book was written, at Vanity Fair. The book is part story behind the images - where the photographers were, how they got the images, etc. and part commentary on how 9/11 was such a widely-viewed event, in part because of advances in digital technology.
There were some interesting stories here - most notably the photographer who took "The Falling Man" photo that many papers refused to run (it later spawned a documentary, which I've seen). The last bit of the book was dedicated to the background and aftermath of "The Shot" - the photograph of the firefighters raising the flag. I had no idea, but there was a ton of fighting and even lawsuits over that photograph, its subsequent licensing, and the money that was made.
This was an interesting read. It was written in 2006, so a lot of references felt dated. He did say, though, that if 9/11 had happened 4 or 5 years later there would have been a lot more video produced, taken by people on their phones, and it would have been instantly transmitted outside via Internet. Those capabilities really didn't exist in 2001.
I had just finished this book when the Boston marathon bombing happened, and it felt really relevant when there were so many immediately iconic photographs taken, and then instantly uploaded so that we were really watching it happen.