Actually, the iPad, or tablet computers in that matter, could open completely new ways of "reading entertainment", since the book could contain not only text and static images, but also other pieces of multimedia or even pieces of software! I am now making up a quick list of what could be possible:
1. iPad books can have video illustrations or sounds embedded.
2. Books can have some taxonomy data. For example, I could see a list of main characters which were presented in the book until the page I have read. I can see their short description, their relationship, or even where are they currently.
3. The text can be "location aware" - for example I am reading some scene, and there can exist an interactive map for that scene, so I will look around myself how it looks like there.
4. There can be some elements of gaming incorporated, like some puzzle or whatever, etc. etc. etc.
The possibilities are really endless here and I am excited to see what the content providers will be able to come up with.