I just finished reading The Great Big Book of Horrible Things: The Definitive Chronicle of History's 100 Worst Atrocities. Fascinating stuff. It starts off chronologically, so in a way it's a history lesson of the most unpleasant times (i.e. most of them), and it covers wars, genocides, man-caused plagues and famines, and the most unpleasant bunch of leaders. It shows a very different side to "civilised" countries too.
Riverting, and highly recommended.