you can buy non-proprietary ebooks that you can read on the kindle without converting them at smashwords, which i wouldn't consider niche, and also at baen. i'm sure there are others. you can also get ebooks for both at project gutenberg - see, there's both nook and epub here
http://www.gutenberg.org/ebooks/28931
not sure why so many people are
so convinced that you can't read any formats in the kindle without converting. you can read non-drm mobi, prc and text files without intervention.