The book is a novelization of sorts, written by pulling info from documents and interviews as well as based on the movie. The writing itself isn't so great, but the story is so incredible that you forgive the bad writing.
It's just incredible to me that these events are REAL, that someone like Pran survived, and that what the Khmer Rouge did really happened. It's really horrifying. The book is incredibly worth reading, if only to learn more about the horror off the time of the Khmer Rouge and learn about the incredible man who still, to this day, works at the NYTimes.