It would be pretty powerful if online textbooks linked together like wikipedia. It wouldn't be too hard for a online textbook company to create a script to find instances of words from their index and link them together. If I could mouse over an important vocab word and have other relevant pages pop up, then online textbooks could be seen as more powerful than paper, rather than as an alternative.
Makes me think about why online dictionaries are so powerful compared too flipping through paper dictionaries.
Personally, i tried to buy online versions of my textbooks, but they required me to download their own client to view the textbooks, which didn't work in ubuntu(I wasn't being stubborn about switching to windows, my textbooks were for programming classes so I definitely needed to stay in my programming environment). I can understand not needing to cater to the small niche of people in linux, but a web based app to view the textbooks through the browser would have been much more pleasant for everyone of any OS.