The book was written by a psychologist who survived the Holocaust camps. The paragraph that always sticks with me goes something like "for sure, the best amongst us never left the camps - they (the guards) would need to pick (kill) a dozen people, and the best wouldn't let it be their friends or family, even if it meant their own death."
"When we are no longer able to change a situation, we are challenged to change ourselves."
"Everything can be taken from a man but one thing: the last of the human freedoms—to choose one’s attitude in any given set of circumstances, to choose one’s own way."
This is also the reason why "Shawshank redemption" is my favorite movie. Very similar concepts.