All the online reviewers are really bad at describing this book, in that they make it seem pretty banal. Basically, they're all like, "This book is about how income inequality is going to increase in the twenty-first century." Which is like, umm, yeah, everyone already believed that. No one thought that the gap between rich and poor was gonna get narrower.
Actually, though, the book is much more thought-provoking than that. It makes a lot of very interesting points. For instance, it posits that the narrowing inequality during the twentieth century was extremely atypical and was a result largely of the way that the two world wars (combined with rampant inflation and the emancipation of the slaves and the advent of decolonization) dramatically decreased the amount of private wealth in the world. Furthermore, it posits that in situations when economic growth is high, income inequality is likely to be decrease (or at least increase by less) than in situations where economic growth is low (or, to be more exact, when economic growth is lower than the annual return on capital.)
Anyway, like I said, I am only a third of the way through it. But it is interesting to think about some of the social and political implications of increasing inequality. For instance, both government and art in the eighteenth and nineteenth centuries were dominated by the aristocracy. All Jane Austen novels, for instance, are about millionaires (or people whose families used to be worth millions). Whereas in the twentieth century, inherited wealth has been a much less prominent feature. We don't read stories about the children of tycoons and we don't dream about meeting them. And, in general, the scions of inherited wealth don't even hold as much political power as they would have in previous centuries.
And, generally, we attribute that to our democratic ethos. However, it could actually be a fluke. Maybe the reason inherited wealth isn't as prominent is just because the World Wars destroyed so much of it. And now that the amount of private wealth has risen to pre-War levels, we'll see a return to a more aristocratic society.