By Max Clarke

Sales of ebooks on’s popular Kindle device are now outselling hardbacks by a factor of two.

This comes as sales of hardbacks are themselves increasing, demonstrating the overall growth in sales for the world’s biggest online retailer.

" customers are choosing Kindle books more often than hardcovers at a rate of more than 2 to 1. This is truly astonishing when you consider that we've been selling hardcover books from for over 13 years and Kindle books for only nine months," said Gordon Willoughby, European Director, Kindle.

Kindle's Whispersync technology saves and synchronises a customer's full library of books, bookmarks, last page read, highlights and annotations across Kindles and free "Buy Once, Read Everywhere" Kindle apps for iPad, iPhone, PC, Mac, Windows Phone, and Android-based devices, so a customer is never without their reading.