There are presses cranking it out in the United States, India and Britain, and the book is in at least its fourth run. Even though the book was already a hit in its native France, it's now taking off among English readers around the world, said Donnelly. She expects that sales in China, Hong Kong and Japan will also soon follow.
Piketty, already widely cited for his work on income inequality, has clearly touched a nerve. The book argues that the underlying mechanisms of capitalism tend towards massive inequality. Piketty argues that the era between 1930 and 1975 -- often hailed for the way in which wealth was broadly shared -- was actually a departure from the norm. That period of economic growth, he says, was the result of unusual circumstances like World War II, a global depression and the government's actions in the aftermath of those events: strong policies raising taxes and increasing regulation. But now, with many of those policies rolled back, societies are reverting back to extreme inequality.
The unlikely bestseller, clocking in at nearly 700 pages, is already serving as an interesting case study for modern book publishing.
One of the hallmarks of the book's success is that it is sold out on Amazon, even though there is a digital version available on Kindle, too. (Disclaimer: The Washington Post is owned by Amazon's founder and chief executive Jeffrey P. Bezos.)
"You can have it on your e-book reader, but that's not the same as having the book," said Donnelly. "I'm not saying this book is a Tiffany's bag, but nobody goes to Tiffany's and buys something and doesn't get that little blue bag. I think there's still some of that about books."
The bestseller is already poised to become the most popular book ever for Harvard University Press. Donnelly predicts it will become akin to another classic for the publisher, John Rawls' "A Theory of Justice."
"It's going in that small cluster of books that makes a real contribution to intellectual and moral discussions in our day," she said.
Donnelly added that even though the book is sold out on Amazon, there are still copies available that can be bought directly from the publisher.
For more on the book, read Steven Pearlstein's review of "Capital in the Twenty-First Century."