It is the economics book that took the world by storm. Capital in the Twenty-First Century, written by the French economist Thomas Piketty, was published in French in 2013 and in English in March 2014. The English version quickly became an unlikely bestseller, and it prompted a broad and energetic debate on the book’s subject: the outlook for global inequality. Some reckon it heralds or may itself cause a pronounced shift in the focus of economic policy, toward distributional questions. The Economist hailed Professor Piketty as "the modern Marx" (Karl, that is). But what is his book all about?