First, H/T Jery Coyne.
Pinker's FAQ on The Better Angels of Our Nature: Why Violence Has Declined is very well worth reading in itself - whether you've broached his huge book or not. He deals very well with furphies, such as the idea that violence is caused by godlessness or that the overall decline of violence across the centuries should be credited to Christianity.
But do the read the book itself (I've now finished it). It is so complex and information-packed that it defies any brief review, and I'm not sure whether I'll be able to find the time to write a review that does it justice. Just go and read it for yourself - really! The scholarly breadth and depth are extraordinarily impressive, the analysis is refreshingly level-headed, and the style is so clear, vivid, and lively that the book is simply a joy to read.
The Better Angels of Our Nature is definitely my number 1 book for 2011. Nothing else I've read this year even rivals it, and I can't see anything surpassing it at this late stage (though I do have a list of books that I'm eager to get to over Christmas and in early 2012 ... a few of them are sitting beside me even now, crying out loudly for my attention!).