Sales of digital textbooks still only account for a small fraction of the U.S. college market. But according to the latest report by the social learning platform Xplana, we have reached the tipping point for e-textbooks, and the company predicts that in the next 5 years digital textbook sales will surpass 25% of sales for the higher education and career education markets. That figure is a revision from the company's report last year, which predicted that 1 in 5 college textbooks would be digital by the year 2014.