LISBON—A book by a Portuguese economist achieved a small feat on its release last month: It instantly topped Portugal's bestseller list, overtaking several diet books and even the popular erotic novel "Fifty Shades of Grey."

The book, "Why We Should Leave the Euro" by João Ferreira do Amaral, has helped ignite a public debate in Portugal about the real cause of the country's economic pain: Is it only the hated austerity needed to secure European bailout loans, or is the euro?

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