has sold more e-books than hardcovers – nearly half again as many – over the last three months, the company has revealed.

For every 100 hardback books sold by the retailer’s US site, 143 Kindle books have been bought over the past three months. Amazon sold more than 3 times as many Kindle books in the first half of 2010 as in the first half of 2009.

Amazon founder and chief executive Jeff Bezos said: “We’ve reached a tipping point with the new price of Kindle. The growth rate of Kindle device unit sales has tripled since we lowered the price from $259 to $189.”

The growth of Kindle books has outstripped wider e-book growth rates, Amazon said, citing figures from the Association of American Publishers, which put the rate at 163% in the month of May and 207% for the year-to-date in May.

Google announced will go head-to-head with Amazon and Apple this year when it launches an e-book service.

Google’s service will launch this summer in the US, although a date for a European service had not yet been revealed.