I've always got an opinion (
), so here's some ideas...
First and foremost, it should be easy to know where books go (internally/external card), with what formats and directories are allowed. Maybe even how to put the files there for beginners. And it should be explained how to open them and close them (to open another or go to a bookshelf screen).
Basic navigation options and font/background choices are also important. And, I guess, how to switch to and from landscape as well as to and from full screen mode. Oh yeah, and how to set a bookmark and go back to it.
Everything else seems to be detail info. But the worst experience is to try software and not even be able to determine what directories the content needs to go in, or how to get to it, or what types of files can be used.