I adore your software. One thing that would be awesome would be the ability to upload quoted portions with annotations to a central website. Then if you were reading and thought your friend would be interested in some portion, you could quote a small bit (maybe two or three sentences), annotate, and upload to a central repository where they could view. This would also be great for personal studying purposes. If you wanted to review notes for a course or topic, you could just view them all in the same place. A folksonomy or taxonomy on tags/categories would be useful, as well as excellently designed time views.

While the legal issues this would bring up are significant, it very clearly fits among the fair use for that text so long as you limit the amount they can quote from a book.

This would propel your application into the limelight as a revolutionary new take on book reading. Give a try! I'd implement it myself if I weren't so busy working and reading books on stanza...