Quick, no-install epub reader: Cool Reader 3.
It's better on Android, but the Windows version will work if you just want a quick epub reader.
I'm looking both for books, and ebook readers.
Regarding books, I'd love suggestions for hard sci-fi, well thought-out urban fantasy, with the specific trait of there being something bigger than the main characters that they have to square off against.
The foe is bigger than you, the struggle is harder than you realized, the merry adventure stopped being merry three bloody battles ago, and you just want to go home. But you can't, because you're the only clever, determined person who's trying to deal with this existential threat. Everyone and everything you know and love will fall to pieces if you don't deal with this. So you need help, you need to think harder about it, and you need to come up with something really clever.
Regarding ebook readers: I'm looking for recommendations for open source ebook reader apps for android with the caveat that they cooperate well with Calibre and allow browsing through metadata.
Edit: Currently using Cool Reader, available in the F-Droid repos.