The number one bestseller in Britain and America right now is Inferno by Dan Brown. Billed as “Dan Brown’s most compelling and thought-provoking novel yet, a breathless race-against-time thriller that will grab you from page one and not let you go until you close the book,” it shot straight to the top.

But in Portugal last month, a book on economics spent a week on top of the list. Why We Should Leave the Euro by Prof. João Ferreira do Amaral now sits at number eight. That’s quite an achievement for what’s usually a niche subject.

But the euro is now a mainstream topic of discussion. “Public lectures, tv debates, newspaper columns and some politicians are starting to explore a question that until recently was confined to university seminars: whether the country has a realistic path to recovery inside the euro,” writes the Wall Street Journal, which drew attention to the new book.

“In 1581 Portugal surrendered to Spain,” says the book. “In 1992 it laid itself at the feet of a European Commission increasingly answering to Germany’s tune. There was no referendum, the voters were never consulted. The Portuguese elites, who hoped to benefit richly from European Structural Funds, cavalierly handed over our currency—and with it our monetary sovereignty. The rest is history.”

Few mainstream politicians agree. But it’s been a year since Portugal held an opinion poll on the euro. Back then, 72 percent wanted to stay and only 20 percent wanted to leave.

That figure may, or may not, have changed. But when an economics book becomes a bestseller, you know something’s wrong with the economy. The riots and protests show a lot of people are unhappy. These book sales tell the same story.

Quitting the euro is no longer an obscure debate in Portugal. The economy is so bad that people are willing to do more than consider radical measures—they’re willing to go out and buy a book about them.

The pressure is building in southern Europe. It’s only a matter of time before it forces Europe to change radically.