Charles Petzold has a post in which he talks about the unique experience of reading a book . In the post, Petzold mentions a device that caused a buzz at a recent O'Reilly conference; the device was a book that somehow embedded a screen for displaying hyperlinked content (from the picture, it looks like it used some kind of flexible screen). Petzold says that while this sounds good initially, it will inevitably lead to more and more ancillary "fat."

I think he's right. I remember that the first few Shakespeare plays I read were from school-provided texts. They had footnotes for virtually every sentence and they were incredibly distracting ("Fardel: A burden.") It was only when I learned to willfully ignore the footnotes that I began to understand why people love Shakespeare. (Exception: ignoring footnotes is not recommended when reading Nabokov.)