This review contains a detailed summary of Pinker's book long argument that humanity has become much less violent over the centuries. It sounds like a fascinating read. Here are some extracts (of the review, not the book):
Pinker thinks that most of what we believe about violence is wrong. To convince us he sets himself two tasks. First, to demonstrate that the past was a far nastier place than we might have imagined. Second, that the present is far nicer than we might have noticed. So to start with we get a litany of horrors from ancient and not-so-ancient history: a catalogue of the unspeakable things that human beings have traditionally been willing to do to each other. This is slightly overdone, since anyone who thinks that, say, medieval Europe was a friendly, peaceable place can't have thought about it very much. Still, it is hard not to be occasionally struck dumb by just how horrible people used to be. The image I can't get out of my head is of a hollow brass cow used for roasting people alive. Its mouth was left open so that their screams would sound like the cow was mooing, adding to the amusement of onlookers.
The real fascination of this book is how we got from being a species that enjoyed the spectacle of roasting each other alive to one that believes child-killers have the same rights as everyone else. ...
At the heart of this book is Pinker's careful, compelling account of why the 20th century does not invalidate his thesis that violence is in a long decline. He makes his case in three ways. First, with a multitude of tables and charts he shows that our view of the century is coloured by presentism: we think it's the worst simply because it's the most recent and we know more about it. If we had equivalent coverage of the whole of human history (how many books have been published about the second world war compared to, say, the Mongol conquests of the 13th century?) we would see that all of it has been scarred by mass slaughters, some of them proportionately even worse than the horrors of the past hundred years.
Second, Pinker argues that the violence of the 20th century is best understood as a series of random spasms rather than part of a trend.