In April, electronic sales outnumbered traditional book sales 105 to 100.

The online retail giant said the sales boost had given Amazon’s US book business its fastest year-on-year growth.

Amazon founder and chief executive Jeff Bezos said he was surprised by how quickly the technology had taken off.

“Customers are now choosing Kindle books more often than print books,” he said.

“We had high hopes that this would happen eventually, but we never imagined it would happen this quickly. We’ve been selling print books for 15 years and Kindle books for less than four years.”

The company added that hardcover sales are still growing, but Kindle books are now outselling them two to one.

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