My friend's father works for a textbook publisher. 35-40% of his yearly sales comes from one textbook. The main thing stopping textbooks from being available electronically are the niche textbook publishers. They are trying tactics like a generous sprinkling of web URLs in the physical textbook (for additional material/discussion). Books are cheap - it's the college bookstore markup and the 'you need this specific revision of the textbook which if diff'd is minor' that equates to extortion.