The author of this little book was a doctor who was selected to help the Nazis with autopsies at Auschwitz, primarily (according to his account) to help them prove their theories that the Jewish people were genetic degenerates (their justification for mass extermination). Nyiszli wasn't a willing participant, of course -- he was only trying to survive from in a no-win situation -- but some of the things he went through or saw done to others, and occasionally some of the things he was a party to, were the kind of awful most of us couldn't imagine unless a record of it were right in front of our eyes.
For instance, Nyiszli talks about the entire trainloads of people he witnessed going directly to the gas chambers upon arrival, and how they were misled into thinking they were getting showers (they were told that to explain why they had to undress -- the Germans wanted their clothes and especially their shoes). He also describes the many autopsies he was asked to do on twins who had been killed right there in camp, and how once he had to do autopsies on a father and son whom he had examined alive 20 minutes earlier.
The Nazis didn't want anyone to know about the gas chambers, and although they tried to keep them a secret, thanks to people like Nyiszli, of course the truth was discovered. Despite the harsh foreward to the book, it's hard to blame Nyiszli for what he was forced to do, because he was only trying to stay alive and keep his family alive -- and if he hadn't been in the position to curry some favor and therefore escape his many brushes with death, he never would have been able to tell his story in this gripping memoir.