Have you ever had the experience of borrowing a book with copious marginalia from the library or a friend, and having those marginalia greatly improve your experience of the book? It happened to me once when I borrowed a copy of The Sun Also Rises from my hometown's public library. Someone who understood the themes and writing style of Hemingway with a level of sophistication I didn't have had made notes in the margins and underlines on specific phrases throughout the text. This person's notes greatly improved my understanding of the text, and I sometimes wished for their previously owned copies of other authors as I moved through my undergraduate English literature education.
Of course, none of us wants just anyone's comments displayed as we are reading - I've borrowed plenty of books with comments in the margins by others that I didn't appreciate seeing (One previous reader of a Jane Austen novel had decided to underline every reference to trees, I assume in an attempt to find a theme. Since I cared not a tiny bit about trees in Austen's text, I found the notes distracting and annoying).
All this makes me excited about a new mobile app, Social Books. Users can share their virtual bookshelf with friends and on Facebook and Twitter. Now when I read a book a friend greatly enjoyed, I can see his thoughts, and be a part of his experience with the book, as I read it. You could have friend groups organized around shared passions and share texts, comments, and links, extending your experience of the text through other readers' (who you find relevant) thoughts.
The uses for education are exciting. Imagine if you could see your classmates' comments in context on a shared text. It could make class discussions much more engaging. Your professor could share the text with the class, with a few notations in text that draw your attention to the areas she especially wants you to pay attention to.
The key to this idea is the social network. Already in the Kindle, readers can see what other readers of the text have highlighted. I almost always turn this off, because other readers don't highlight what I would have highlighted, and all other readers of a text are not relevant to me. But if I can read 20 Economic and Demographic Factors Driving Online and Blended Program Enrollments by Betts et al and see the comments of colleagues who are also involved in online higher education, the text is enhanced in a way that is relevant specifically to me.
I can't wait to get the app. I hope my friends and colleagues do as well.