From the press release:
More than 18% of new textbook revenue in the Higher Education and Career Education markets will come from digital textbooks by the year 2014, according to a new study by Xplana. By 2014, annual revenues from digital textbooks will exceed $1 billion. This shift in sales will cause the publishing industry to overhaul the traditional textbook production processes.The study, "Digital Textbook Sales in U.S. Higher Education – A Five-Year Projection," projects annual sales growth and market trends for digital textbooks over the next five years. It is based on private surveys conducted by Xplana and its parent company, MBS Direct, as well as other industry and market research.
You can download the full report here.
My first reaction was that I doubted that this would happen, but on reflection I decided that 18% in 5 years is probably a conservative estimate. It's almost a certainty that the digital book market as a whole will hit 18% before that date.