This book is very short, but it undoubtedly is one of the heaviest and most dense I've read this year. It includes absolutely horrendous descriptions of war crimes, so check the content warnings below. I will not refer explicitly to these in my review, but I wish I'd had a note at the beginning of the book. 
The Man Who Ended History is an absolutely fascinating little book in many ways. It reads like the transcription of a TV documentary juxtaposing interviews, voice-overs and textual descriptions of visual material presented on screen. This apparent distance helps tell about the atrocities committed during the China-Japan War in the 1930s, but no distance can really prepare you for the horrors of the Japanese camps. The most chilling part may be that most of the accounts presented here are taken from historical sources and have nothing fictitious, despite this book being marketed as Sci-fi. 
The premise is that a Japanese-American physicist, a woman, and a Chinese-American historian, a man, have developed a tool to witness historical events as if one were transported then and there. From then, the documentary focuses on the ethical and philosophical issues raised by this endeavour, since the historian decided to give this opportunity to families of victims rather than scholars, and that each "time-travel" destroys the possibility to visit each time period a second time.
There is a lot to talk about from such a small volume. One huge caveat I must raise, however, is that once again the author is determined to minimise the role of women, as he'd done in The Grace of Kings. First in the title itself : the man of the title is the historian, but without the woman physicist none of the experiment would have been possible. Secondly, the form is a documentary which means that everything is presented very neutrally. Then why is there an italicized description of the physicist making comments on her beauty?? It has absolutely nothing to do here, and yet it's the very first sentence of the book. This baffles me. I know after The Grace of Kings I'd said I'd stay away from Ken Liu in the future, but this book had been heavily recommended and it sounded too interesting to pass. Which it is, undeniably. 
Additional TW for mutilation and vivisection.