This is a freakin intense book. I can't really say which other book made me as physically uncomfortable while reading. In terms of fiction it may very well have been The Painted Bird, in terms of non-fiction it could be Escape from Camp 14.

The Rape of Nanking is in its first half a pretty detailed account of just this: How the Japanese forces occupied Nanking during WW2 and proceeded to kill, torture and rape the civilian population, making it one of the largest atrocities of war ever committed. The accounts are pretty intense and are told from three perspectives: The Japanese aggressors, the Chinese population and some German & American foreigners living in Nanking and how they helped saving the Chinese population. Amongst others, the story of John Rabe is told, a Nazi living & working in Nanking, who became the 'Oskar Schindler of China', saving thousands of people.
Did I already say, it's pretty intense and I recommend taking breaks while reading.

The second half of the book goes on to explain why you most likely never heard of Nanking (I for one never heard about this before reading this book), how the different nations dealt with the aftermath and also what became of some of the people who survived the Rape of Nanking. Which for the most part is also pretty frustrating and bleak. Just in case you should have believed that everything will be fine in the end.

Recommended for: Everyone should suffer through this great book, lest we forget.