Exhibiting a shift in the popularity of digital reading material, Amazon, the online retailer, says that it now sells more e-books than hardcover books. The figures show Amazon's dominant position in the e-book market.

E-book sales rose to overtake those of hardbacks during the past three months. During the period, Amazon sold 143 Kindle books for every 100 hardcover books. The disparity was greatest in the past month, when the ratio increased to 180 e-books for every 100 hardcovers.

Sales for the Kindle reader had grown threefold since Amazon cut its price to $189 from $259. The price reduction was introduced to compete against Apple's iPad. Amazon did not release specific sales figures for e-books or the Kindle. However, it says five authors have sold more than 500,000 copies.

In spite of an early lead in sales of both e-readers and e-books, Amazon faces increased competition. Rivals such as Barnes & Noble, Borders and Sony have released new e-readers and e-book stores in hopes of securing a stronger position in the market.