You may use FBReader, abbreviation of Free Book Reader.
It is a small application and very fast.
It does the job of opening eBooks just fine.
Installing FBReader is very easy because it is available in most of the Linux repositories by default.
To install FBReader in Ubuntu and respective Linux Mint and elementary OS versions,
use the following command in a terminal:
$ sudo apt-get install fbreader
Downloading fbreader from snap store on ubuntu
$ snap install fbreader