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.
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!
> Includes support for mobi and epub
Curious. I didn't know that was there.
> as well as TIFF,
The last time I've tried it didn't seem to support multi-page TIFFs
> CHM
Very curious. Never seen a good alternative CHM viewer (although seen apps advertised as such numerous times).
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).