When a book about economics rises to the No. 1 spot on the bestseller list, it says as much about society as it does about the book. That’s why the explosive rise of “Capital in the Twenty-First Century,” by Thomas Piketty, is so revealing and why the book will become a self-reinforcing phenomenon likely to carve a deep mark in the political landscape.

Piketty’s work, on its own, is an important and impressive accomplishment. But the fact that it has been welcomed so enthusiastically by such a wide spectrum of the population proves that it has hit a nerve. ...