As for students needed to interact with textbooks differently than novels, that is very true. To be an effective e-textbook, students will need to have the ability to interact with the texts to do things such as take margin notes, bookmark, underline, highlight, etc.
As a side note, I'm always a bit concerned when data about ebooks from the National Association of College Stores. I'm not surprised that they report that less than 3 percent of textbook sales are digital books, but what else could that mean? It doesn't necessarily include the rented ebooks, or ebooks that students get access to with printed text, it wouldn't include at all open source texts (such as I use with my undergraduate class) as there is no sales associated with such books.
As the e-book market explodes, publishers and educators debate why e-textbooks lag behind -- and what they should even look like. Campus Technology (3/1/11) http://campustechnology.com/Articles/2011/03/01/Can-Tech-Transcend-the-Textbook.aspx?Page=5&p=1