Here's a nice summary of the state of digital textbooks and where they are heading. If any part of the publishing business needs to go digital it is this one. Too many school and university classes sit reading textbooks that are sometimes over 10 years old and how reliable are they as sources? Digital material can be updated continuously and kept relevant but they're unlikely to be as profitable as the printed versions have been for so many years.
As ever this graphic is probably on hundreds of blogs by now but good things are meant to be spread. Just click on the graphic to see a larger version.