E-book sales more than doubled in the US Y-o-Y in January 2011

"E-books and downloadable audio books continue to grow in popularity according to the January 2011 sales report of the Association of American Publishers. Figures for the first month of the new year show that E-book net sales increased by 115.8% vs January 2010 (from $32.4 Million to $69.9M). Sales of Downloadable Audio Books also rose by 8.8% vs the previous year ($6.0M to $6.5M). As AAP reported last month in its December 2010 monthly report and full 2010 analysis, E-book sales have increased annually and significantly in all nine years of tracking the category."
Source:  Press release from the AAP, reported by Engadget, 20th March 2011