I was working on a post today about ebook apps and I noticed an error in my assumptions which I thought might interest you.
If you'd asked me this time last year, I would have told you that there were certain types of books which weren't practical to convert to ebooks. For example, coffee table books with their large images, complex formatting, etc, would never work on the Kindle or other epaper ereaders. (They'd work on the iPad and tablets, but this time last year tablets and the iPad weren't as important as they are now.)
I might have been wrong then, but I an certainly wrong now. It's a lot easier to make coffee table books for the iPad now. You can now use the Baker and Laker frameworks as the basis for your app, and Apple have enabled a full screen option for iBooks. Before this new option, iBooks forced you into a basic page layout which precluded complex formatting and full screen images. Now you can do a full screen image like you would see in a coffee table book.
If you'd like to see an example of a coffee table book on the iPad, check out Liz Castro's Barcelona: Beyond Gaudi. I can't find it in iTunes just yet, so you'll need to get it.
My point here is that if there's a type of book that you believe can't be made into an ebook, wait 6 months then it will be doable. Wait another 6 months and it will be common.