THAT left- and right-leaning Americans read different books might be the least surprising fact about publishing. After all, they live in different places, eat different food, listen to different music and, of course, consume different kinds of news. All these reinforce one another; increasingly, progressives and conservatives simply do not know each other. And an analysis of book sales on Amazon, done for The Economist by Valdis Krebs, a data scientist specialising in network analysis and visualisation, bears that out graphically (see chart).