I take projections with a heaping lump of salt, but this is interesting:
Considering current digital textbook sales increases, and basing our assumptions of the favorable evolution of factors contributing to increased digital textbook availability and access, we project the digital textbook market to surpass 18% of combined new textbook sales for the Higher Education, and Career Education markets in the U.S. by 2014. Overall digital textbook sales will increase 100% year-over-year in 2010, and continue to grow at rates of 150% and 120% respectively in 2011 and 2012. As publishers struggle with the eventuality of transforming their product models to digital-first, and as they adjust their sales efforts to address the growth in the digital market, we expect a certain amount of churn and an adjustment to sales growth in 2013 and 2014. In those years, digital textbook sales will increase 90% and 80% respectively.