Every so often a book comes along that literally changes the way I think about the world. Thomas Kuhn's Structure of Scientific Revolutions was probably the <cough> paradigm case for me.
Steven Pinker's The Better Angels of Our Nature: Why Violence Has Declined is one of these books.
His core thesis, supported by a huge array of data and documentation, is that violence has declined dramatically over time — not always smoothly, not always consistently, past performance does not guarantee future results — but there's a clear downward trend.
The kernel of insight is one of those really-obvious-in-retrospect ideas that changes perspective on a huge amount of history: if you look at conflicts and categorize them not by how many people they killed, but by how many people they killed per capita (i.e. divided by the world population at the time), then generally speaking, the fraction of people who die in armed conflicts has been getting smaller over time. A lot smaller.