Life is getting less violent. Really.
The first portion of the book deals primarily with the statistics and evidence that shows how violence has declined in the course of recorded human history; and the facts do show a dramatic decline of violence over the centuries. While reviewing the statistical numbers Mr Pinker also attempts to find reasons for the declining violence, looking at many different theories that have been proposed over the years.
The two chapters prior to the final summary/conclusion discuss the psychological science behind some of the reasons for the decline in violence and how violence and non-violence are worked out in the human brain. I found this particular portion of the book the most fascinating, and I can imagine that there is much more that could be expanded upon.
This book is a rather large work and did take me some time to get through all of it. At times I had to set it down and read something else, coming back to it after I was ready to take on a new section. I recommend taking it in portions due to the voluminous amount of material covered. Overall this book is a well done piece of research and I highly recommend it; I would even go so far as to say that this is a must read in 2012.
One side note: do to the nature of the subject matter, some portions of the book describe things that some people may have trouble reading. There simply is no way to describe what medieval torture was like without a few graphic details; and one does need to understand exactly how gruesome it really was to understand how far we have come.