The Better Angels of Our Nature by Steven Pinker.

Not because of the actual topic, although that's important too; but because of the first 150 or so pages, where Pinker starts by saying something like: you won't believe me about how violence has declined unless I first disabuse you of the notion that the past was nice. He then describes how truly awful pretty much everyone and everything was in the past. It was an eye-opener.

Those who romanticize the past (which is most people) do themselves a grave disservice.