Branko Milanovic, a former economist at the World Bank, called it “one of the watershed books in economic thinking.” Paul Krugman, winner of the Nobel in economic science and a columnist for The New York Times, wrote that it “will be the most important economics book of the year — and maybe of the decade.” Remarkably for a book on such a weighty topic, it has already entered The New York Times’s best-seller list.

“Capital in the Twenty-First Century,” with its title echoing Marx’s “Das Kapital,” is meant to be a return to the kind of economic history, of political economy, written by predecessors like Marx and Adam Smith. It is nothing less than a broad effort to understand Western societies and the economic rules that underpin them. And in the process, by debunking the idea that “wealth raises all boats,” Mr. Piketty has thrown down a challenge to democratic governments to deal with an increasing gap between the rich and the poor — the very theme of inequality that recently moved both Pope Francis and President Obama to warn of its consequences.

Mr. Piketty — pronounced pee-ket-ee — grew up in a political home, with left-wing parents who were part of the 1968 demonstrations that turned traditional France upside down. Later, they went off to the Aude, deep in southern France, to raise goats. His parents are not a topic he wants to discuss. More relevant and important, he said, are his generation’s “founding experiences”: the collapse of Communism, the economic degradation of Eastern Europe and the first Gulf War, in 1991.

Those events motivated him to try to understand a world where economic ideas had such bad consequences. As for the Gulf War, it showed him that “governments can do a lot in terms of redistribution of wealth when they want.” The rapid intervention to force Saddam Hussein to unhand Kuwait and its oil was a remarkable show of concerted political will, Mr. Piketty said. “If we are able to send one million troops to Kuwait in a few months to return the oil, presumably we can do something about tax havens.”

Would he want to send troops to Guernsey, the lightly populated tax haven in the English Channel? Mr. Piketty, soft-spoken, barely laughed. “We don’t even have to do that — just simple basic trade policy, trade sanctions, would do the trick right away,” he said.