Just read through the thread and I have been doing the same thing. Hunting around for a format to store all of my books in that is easily converted to others. I think I've settled on leaving them as oeb.
https://wiki.mobileread.com/wiki/Open_eBook
FBReader will read these, as a zip file, and they are also easy to convert back into a lit or a mobi pocket file or epub, its also easy to convert the html files into a plucker file which is what I use on my palm devices to read books. Never could get any of the mobi versions to work on my Sony Clie's without crashing.