French economist Thomas Piketty smiles during a meeting at the National Assembly on March 13, 2013 in Paris. AFP PHOTO / FRED DUFOUR (Photo credit should read FRED DUFOUR/AFP/Getty Images)

The number-one book on Amazon.com isn't a guide to green juice or an erotic romance novel. No, the top seller on Amazon right now is a 700-page book, translated from French, about rising inequality and the state of modern capitalism.

In what may be a hint of widespread anxiety about the foundations of the U.S. economy, Thomas Piketty's "Capital in the Twenty-First Century," which is currently sold out on Amazon, is beating books like the science fiction mega-hit "Divergent" and the Pulitzer Prize-winning novel "Goldfinch." Also in the top five on Amazon is Michael Lewis's latest, "Flash Boys," about high-frequency trading on Wall Street.

Piketty's book, which is also a New York Times best-seller, challenges the conservative economic theory of trickle-down economics, or the belief that a rising tide lifts all boats. In Piketty's view, backed by centuries of data on wealth and economic growth, the typical outcome of unfettered capitalism is rising income inequality. Piketty says the world's biggest economies have to do something, like impose a global tax on capital, to stop it. As Piketty said in an interview with HuffPost Live last week, income inequality is only getting started, and this century could look a lot more like the deeply unequal 18th and 19th centuries than the more-egalitarian 20th.

The New York Times has called the book a "blockbuster." Vox's Matthew Yglesias described it as "the most important economics book of the year." Nobel Prize-winning economist Paul Krugman hailed Piketty's work as revolutionary, crediting him and his collaborators with getting us all to care about the rise of the so-called "one percent."

Just be warned: If you're thinking of hauling Piketty to the beach, you're going to need a sturdy bag.