You should probably read this article by Roger Sperberg at the group blog Electric Forest. He works through a few of his thoughts on ebooks, and in the comments section mentions a follow-up article. I'll reserve comment until I see the follow-up article and I have a little time to think about this in more depth.
Quickly, though, I wonder if he's aware of Logos Bible Software. That is, if he has seen it and not just heard about it. Logos strives to reproduce the printed page as much as it makes sense in an electronic environment while adding features appropriate for an electronic environment (the Libronix Digital Library System, in this case). These enhancements are primarily in the realm of hypertext referencing (so, click on a Bible ref, or a Josephus ref, or a reference to 'page 347' and go there), topic indexing, and (increasingly) in distinguishing different fields of information for searching purposes.
Some resources take this quite far. The morphologically tagged editions of the Hebrew Bible and Greek New Testament have all sorts of data stuffed in there, associated with specific words. This would never work in print, it only works electronically — much like Mr. Sperberg's chess example only works electronically and doesn't work in print.
Other resources have a relatively high degree of interaction. One recent example is Moody's AM Bible Courseware (be sure to check the video at the bottom of the page) which is powered by the Libronix Digital Library System. The books are delivered as books, they are cross-referenced with the larger Logos Bible Software library. And yes, there are tests. The realm of distance education is one area where great strides have been made in the area of ebooks (even 'free on the web' stuff — check out MIT's OpenCourseWare).
There are many things that could be done electronically that don't occur in Logos books. I like to describe these sorts of things as a sort of "multimedia extravaganza". It is all in accordance with Brannan's First Law of Electronic Book Design:* Just because you can doesn't mean you should.** Just because one could make an edition that animated page-flips doesn't mean that one should do it, no matter how cool someone might think such a thing would be. Instead, the goal is a usable edition familiar to those who use printed reference books with enhancements that fit the platform and the data — not clickie-clickie eye-candy or projects that attempt to convert publishers into movie producers.
And I'm not saying that Mr. Sperberg is advocating clickie-clickie eye-candy. His examples are reasonable, for the most part.
Anyway, I'd better stop now. I'll see about posting more after Mr. Sperberg's follow-up is posted.
Update (2005-05-16): Roger Sperberg has his follow-up posting online: Can our libraries be digital if the books are not? Be sure to check it out.
Update II (2005-05-16): I've been having a good email discussion with Murray Altheim and Roger Sperberg from Electric Forest since posting this article. Thanks to the both of them for their interest and willingness to discuss these things. I know I'm enjoying it. I hope to post some more information about books/ebooks and reading platforms based on some of the discussion in the very near future, depending on the time I have available in the next day or so.
* No, I'm not going to list out all of my Laws of Electronic Book Design. That would require me to actually codify them.
** Let's not forget the corollary to this law: Just because you should doesn't mean you can.