Stephen RamsayTIL about Zathura, which is "a highly customizable and functional document viewer" that is written in C, uses a plugin architecture, and understands vi-keystrokes. A little niche -- okay, a lot -- but man, this thing is fantastic.
I hate most ePUB readers. They're either too bloated (Calibre) or they're nicely minimalistic but don't, um work (Foliate, at least for me). This one is just perfect.