The Better Angels of Our Nature: Why Violence Has Declined. It's not a perfect book, but it puts forth some pretty good ideas as to why violence is declining.
As for the uptick in violence in the mid-late 20th century, leaded gasoline is the culprit. The evidence for this is overwhelming. I won't go into the details in this post because there are a couple articles on the subject that explain it better than I ever could.
How Lead Caused America's Violent Crime Epidemic - Forbes
America's Real Criminal Element: Lead | Mother Jones