The AAP has been tracking ebooks since 2002. That year, ebooks represented 0.05% of all trade publishing revenues. To get to the current 23% number, the biggest gains were made in 2009, 2010 and 2011, the years immediately following the 2007 launch of the Kindle. In 2008, ebooks were 1% of publisher revenue. In 2011, they were 17%. Those were the years of triple-digit growth numbers, a trend publishers thought would continue until ebooks were at 50% of revenue or more. But in 2012, according to these new numbers, growth in ebooks has hit an inflection point in the U.S.
via Ebook Growth Slows in 2012 to Only 41%; What Does It Mean for the Publishing Industry? – Forbes.