If you enjoy history at all, Laurence Rees has some good books on WW2:
War of the Century: When Hitler Fought Stalin
Horror in the East: Japan and the Atrocities of World War II
Absolutely could not put those two books down.
Another good read is "Machete Season: The Killers in Rwanda Speak" An account of the Rwandan Genocide by the author Hatzfeld. She is a journalist. This book looks at the killers themselves, and features testimonies of ten men, now in prison, with the attempt to understand their state of mind, and the forces behind the atrocities.