There's an
article on US News that discusses the current status with e-textbooks. It mentions various sites like textbookrevolution.org, textbookmedia.com and is extremely bullish on the future of digital books:
"As the publishing landscape changes, academics are already thinking ahead to the next generation of print textbooks. A group of 50 leading teachers, technologists, and scientists studied the problem for the National Academy of Sciences last summer. They concluded that the next texts will look more like guidebooks travelers use to explore new cities than textbooks. They predicted that they will be far slimmer, customizable, and more challenging than their current incarnations."
So far I can tell digital textbooks have failed even worse than general e-books by publishers introducing ridiculous DRM measures (e.g. expiring content) and offering absolutely no price advantage to the student.