Pinker’s book provides a lengthy and comprehensive argument that levels of violence have steadily reduced over the course of human history. Having presented the facts that support the argument, he then discusses why this might be so. It is timely admonishment for those Cassandras who believe, in a nostalgic haze, that everything is going to hell in a hand basket. Happily, though, he does not overplay his hand and pretend that we can be complacent about the future. It is rather a case of having perspective.
Link to book: https://www.amazon.co.uk/Better-Angels-Our-Nature-Violence/dp/B00ACT3U1A/ref=sr_1_16?s=books&ie=UTF8&qid=1495019061&sr=1-16&keywords=The+better+angels+of+our+nature