Hey Jason, as someone you personally talked into running an ultra within the past week basically (even though I didn't realize you'd done so at the time) by the end of the year...here's what I'd really like to see:
First, I would love a section that instead of trying to cover everything, dedicate a section to teaching you "how to learn." That may sound silly, and I don't mean a long list of good links either, while that's useful, it's really not book material and gets dated very, very quickly. Several of the many books I've read recently make sweeping, general recommendations like, "You should volunteer for an ultra first" or "Pacing for someone is a great experience if you're planning an ultra yourself." None of these books address the question of "Ok, so my spouse/sibling/friend isn't an ultra runner already, how do I find out how to do this?" For someone who's pretty Internet savvy, it isn't impossible to find this information, for someone who isn't, the task might be a lot more difficult.
You simply cannot write a book that will teach someone everything they need to know (or should know, or could know to do their absolute best), but providing just enough information to be useful and concentrating on what I said above I believe will make your book something truly unique. For what that's worth.
There's also a bullet point list on your old site (that hidden stuff we talked about) that talks about how to select your first race. It talks about number of aid stations (saying more is better, but what is average, what is below, etc). It talks about loops vs straight, etc. While that list is great, you could dedicate an entire chapter on it for new ultra runners. While there is a ton of great information on your site, a lot of it overlaps with other sites. That list stood out to me as something I hadn't (and still haven't) seen anywhere else.
Lastly, and it's just my .02, hydration and caloric intake issues are so incredibly important for ultras, but EVERY book covers that information in great detail. I find myself slogging through it in almost every book I read for a number of different reasons. First, theories change so rapidly that it's nearly impossible to tell when you're reading (most people who read your book will have no idea the age of the book, most people just don't care about that sort of thing, in my experience) if the information is out of date or not. Second, what isn't subject to being dated (and a lot of it wouldn't be) I've already read about in the previous three books I just read (as an example, I've read way more than that personally). Just weigh the pros and cons of that, since it's so important as to be in every book, it certainly requires some coverage...but also remember that people are looking for new information. Going hand in hand with the above, teaching someone how to learn might be a better approach, rather than trying to cover specifics.