This is a good article on eBook design, as informed by Bible-study software. Bible-study software is a niche that every software entrepreneur would do well to know about. It’s a big market and it attracts enthusiastic and diverse developers who produce innovative features (sometimes worthy, mostly not). But unlike, say, videogames, everyone works from the same base material and is more prone to focus on the user experience than the technology at hand.
It also would be a smart area to look at for platform adaptation and marketshare, since one can be assured that any new platform is going to get bible-study software early in its production and yet is unlikely to involve a statistics-distorting breakout hit on any specific platform.