A little background…
In June 2010, Amazon announced that it had sold 143 ebooks for every 100 printed books in the prior three months -- a figure that included hard copy books that didn't have a Kindle version, not to mention that the store offered only 630,000 Kindle books at the time compared to its physical library of nearly two million titles.
At the time, Amazon said it was astonished considering that it had been selling hard cover books for 15 years, and Kindle books for only 33 months. The company launched its first Kindle e-reader in November 19, 2007.
The shift occurred even quicker on Amazon UK, which introduced the Kindle in August 2010 and by May of the following year ebooks outsold printed books at a rate of more than two to one.