Now that a variety of ereaders have finally legitimized the concept (a Kindle addict myself), the opportunity for untold mischief abounds.
1) One Click. Make everyone buy a copy of the latest entry in the Twilight series.
2) Propaganda. You've longed suspected Mrs. Pelosi needs
a refresher on Atlas Shrugged. Send it to her.
3) Subversion. Imagine subtly changing text in books from the original, inserting your own concepts, confusing Shakespearean quotes from multiple plays, or scrambling legal paragraphs.