Where Are All the iBooks Textbooks?

I have been doing a bit of research into the iBooks textbooks ecosystem. I have a few topics in mind for potential textbooks that I may publish. These would not really be K-12 focused but more advanced technical topics. I have been watching the iBooks store and I just don't see much in the way of textbooks being published outside of the high school market. But why not?

Apple seems to keep their publishing agreements very opaque and perhaps under NDA, but at their education event in February they did announce that textbooks in the iBooks store would be no more than $14.99. This is not a lot of money for a textbook and perhaps that is keeping the mainstream publishers off the platform, but what about independent content creators such as myself? Clearly I would rather charge more than $14.99, but it wouldn't be a dealbreaker.

Is there a review process that's keeping content out of the store? I can understand Apple wanting to keep the quality of the experience high, but it does not seem logical that they would want to dissuade creative early adopters from embracing their new textbook technology just because their chosen subjects are not what Apple had in mind. I'd hate to invest the time and effort it would take to generate a high quality interactive textbook only to find that it was rejected from the store. If anyone has any experiences along those lines I'd be very appreciative if they are willing to share them.

On a separate but related vein, iTunes U has been out for at least 6 years now. It got a major overhaul early this year, taking it from a source for video lectures to a true publishing platform for online education. Yet it is restricted to accredited institutions of higher education. Again, I can understand Apple wanting to keep the quality of the content high. And I don't necessarily think that the public iTunes U site should offer more proprietary courses. But clearly there is a huge market in professional education for a tool like iTunes U. I have searched to see if I could find a white label version or an iTunes U server application, but not only did I not find anything but I didn't find much call for one. I can't imagine I'm the only one to see the commercial appeal of a high quality iPad-based training tool. It could be possible to replicate the iTunes U features in a third party app, but the developer ecosystem is heavily disincentivized to write one since Apple would no doubt either bar it from the iTunes store or co-opt the idea and integrate it into the official iTunes U offering.

N.B. This really isn't meant to be an Apple site. I just write about what's on my mind, and apparently the last few months have been a number of Apple-related topics.