Back in 2001 two French economists, Thomas Piketty and Emmanuel Saez, circulated a seminal research paper (formally published two years later) titled “Income inequality in the United States, 1913-1998.” They used data from income tax receipts to do two things that you can’t do with standard data on the distribution of income, which come from household surveys. First, they gave us a portrait of the economic stratosphere — the incomes of the now-famous 1 percent. Second, they gave us historical depth, reaching all the way back to the late Gilded Age.

The picture that emerged was startling to those who still clung to the notion of America as a middle-class society, or who thought of rising inequality as mainly a tale of divergence between blue-collar workers and a fairly broad elite, like college-educated workers. Mr. Piketty and Mr. Saez showed that the actual story of rising inequality since 1980 or so was dominated not by the modestly rising salaries of skilled workers but by gigantic gains at the very top — a doubling of inflation-adjusted income for the top 1 percent, a quadrupling for the top 0.1 percent, and so on. They also showed that these surging top incomes had more or less reversed earlier movements toward equality, that the concentration of income in the hands of a small minority was back to “Great Gatsby” levels.

It was a landmark piece of research that has had a major impact, not just on economics, but on political science too, for the fall and rise of the 1 percent turns out to be closely correlated with the fall and rise of political polarization. And last year, of course, Mr. Piketty made a huge splash with his magnum opus, “Capital in the Twenty-First Century,” which both exposed the startling facts about inequality to a wide audience and made a disturbing case that we are well on the way to re-establishing “patrimonial capitalism,” a society dominated by oligarchs who inherit their wealth.

“Capital” is a powerful, beautifully written book (wonderfully translated by Arthur Goldhammer). It is also very big and quite dense, and there’s reason to believe that many people who bought it didn’t get very far in their reading. So it would be really helpful to have a short-form exposition of the essentials of that masterwork.