Perpetrators: The World of the Holocaust Killers

Perpetrators: The World of the Holocaust Killers by Guenter Lewy, 195 pages.
Lewy seems to be wanting to get to some definitive answers as to the motives of the perpetrators of the Holocaust, without going too far in any one direction. The book is massively sourced and footnoted,  with  over 30 pages of source material, and 156 footnotes in the second chapter alone. At the same time, with only 136 pages of text including the introduction, he doesn't seem to leave himself a lot of room to explore his several topics in depth.
He covers the concentration camps, deaths by shootings, the development of more efficient ways of killing (seeking to avoid stresses on the killers), the hunt for and trials of those responsible, all while trying to give individual and group motivations for killing thousands and thousands of people. An interesting, but ultimately unsatisfying book.

Hanns and Rudolf: the True Story of the German Jew Who Tracked down and Caught the Kommandant of Auschwitz / Thomas Harding 348 pp.

Hanns Alexander escaped Berlin in the early 30s to England, served in WWII, and afterward served Britain's War Crimes Group as an investigator.  In this role, he tracked and captured Rudolf Hoess, Auschwitz' Kommandant, who oversaw the murder of some 3 million people, most of them Jews.  Hoess is not to be confused with Rudolf Hess, Hitler's Deputy Fuhrer.

Harding is Hanns' great-nephew, and he has put together a highly satisfying piece of storytelling.  In alternating chapters, he tells the story of the two men's lives from infancy forward. The book jacket states that he is a former documentary filmmaker, and the skills of that trade are obvious here:  the book is informative and well-researched, but Harding keeps his focus on the reader's experience, and moves the story at an expert pace.  Hoess' capture and trial are suspenseful, and the details of Hanns' emigration interesting, but it is Harding's style in writing Hoess' story that shines.  He has a calm,  almost modest tone as he describes the life of a seemingly ordinary German boy once destined for the Church, who later became a loving husband, father, and mass-murderer.