Ordinary Men by Christopher Browning
It's about a group of 500 police officers from Nazi Germany belonging to Reserve Police Battalion 101 who, during the Holocaust period, were tasked with the killing and deportation of thousands of Jews in eastern Poland. These were mostly middle-aged, working class men from Hamburg, who were raised before the Nazis came into power and who, to put it simply, did not know what they were getting themselves into, and mostly had no active interest in the elimination of the Jewish population. And yet, despite this, and despite being under no total obligation to carry out the killings (i.e. they could opt out if they wanted to without punishment), the battalion as a whole murdered a minimum of 38,000 Jews between 1942/43, where at least 90% of the men carried out at least one killing.
This is my second time reading this book and it is perhaps the most devastating book I've read. It is deeply troubling to think how prone human beings are to fall into conformity, even when faced with the liquidation of all the Jews in a town ghetto, as in their first massacre at Jozefow where they murdered at least 1,500 Jews. It is very easy for us in the 21st century to treat the Nazi perpetrators as some "other" and to imagine that if we found ourselves in the same situation, we would be the white knights who were hiding the Jews and would know better. But the thesis of the book is that the vast majority of these police officers were not bloodthirsty killing machines who honestly believed in what they were doing, but ordinary men who were carrying orders - some of whom were previously involved in the Hamburg met police force, some who had ended up there because it was their alternative to being conscripted to the army - and what Nazi propaganda they had received during their training was actually quite inconsequential to their actions (as Browning argues at least, in the concluding chapter), and that at any rate it was not enough to turn them into killers.
Overall, I see this book as essentially a long, difficult look in the mirror, because, as the final sentence reads: "If the men of Reserve Police Battalion 101 could become killers under such circumstances, what group of men cannot?"