Design varies. Not just in its quality and implementation but also in its purpose and kind.
In my mind, visual and interactive design should be treated as separate but interdependent problem domains.
What follows are two hierarchies of e-book design, outlines that climb from the trivial and decorative to the integral and necessary.
The visual design examples are books that I happen to have on my desk. The interactive design examples are, for the most part, major titles from interactive media history.