When it comes to an eBook reader, the choices are limited. The market is dominated by Amazon’s proprietary Kindle along with a few other options like Kobo, Nook and Onyx.
An interesting news for open source enthusiasts is that a developer, Joey Castillo, is working on creating an open
IIRC it only suports plain text files / Markdown rn. Not supporting EPUB is a non-starter for me. I use my Kobo right now and love it. If they add EPUB support i will heavily consider building one.