Okular's my favourite PDF reader -- I install it even on Microsoft Windows machines I have to use -- but it doesn't read .epub or .mobi files unfortunately.
Hot dang you're absolutely right - I'd never discovered this package before:
okular-extra-backends
Includes support for mobi and epub, as well as TIFF, CHM, Markdown etc. Weirdly the .epub and .mobi are quite slow to open compared to PDF, but I'm happier now for the suggestion to go hunting for this - thank you!
Yeah, it's well hidden - I'm sure I've looked for okular support for mobi & epub a while back, mostly because 'ebook-reader' that comes with calibre is a bit pants.
Must have been quite a while back, I guess.
On Debian unstable I'm seeing okular-extra-backends (as mentioned) along with packages for ODP and ODT, as well as an okular-mobile package (postscript, dejavu, DVI, comic books, fictionbook, Plucker, etc.
I've just tried this with an Asterix comic in .cbr format - no joy.
Works a treat with a Deadpool .cbz file though.
For CHM, and I've not [ found / been forced to acquire for want of a better format ] any of those since 2008, kchmviewer is pretty fine (I'm a KDE user so it fits nicely, though like most KDE apps it only wants some KDE / QT libraries to run on your preferred DE).