Fedora 33 : Sigil software.

Sigil is a ePub editor for Linux and omes with powerful features like UTF-16, EPUB 2 spec, and limited EPUB 3 support.
The complete control over directly editing EPUB syntax in Code View and Table of Contents generator with multi-level heading support and metadata editor.
The user interface translated into many languages and spell checking and many plugins available.
These command will show you how can search and install this software:

[root@desk mythcat]# dnf search Sigil
Last metadata expiration check: 0:54:58 ago on Wed 16 Dec 2020 11:01:54 PM EET.
======================== Name & Summary Matched: Sigil =========================
FlightCrew-sigil-plugin.x86_64 : Sigil FlightCrew epub validator plugin
sigil-doc.noarch : Documentation for Sigil ebook editor
============================= Name Matched: Sigil ==============================
sigil.x86_64 : WYSIWYG ebook editor
[root@desk mythcat]# dnf install sigil.x86_64 
Last metadata expiration check: 0:55:32 ago on Wed 16 Dec 2020 11:01:54 PM EET.
Dependencies resolved.
================================================================================
 Package                     Arch       Version               Repository   Size
================================================================================
Installing:
 sigil                       x86_64     0.9.14-7.fc33         fedora      3.5 M
Installing dependencies:
 minizip                     x86_64     2.10.2-1.fc33         updates     112 k
 python3-cssselect           noarch     0.9.2-16.fc33         fedora       38 k
 python3-cssutils            noarch     1.0.2-9.fc33          fedora      270 k
 python3-regex               x86_64     2020.11.13-1.fc33     updates     345 k
 zipios++                    x86_64     0.1.5.9-25.fc33       fedora       78 k
Installing weak dependencies:
 FlightCrew-sigil-plugin     x86_64     0.9.1-21.fc33         fedora      336 k

Transaction Summary
================================================================================
Install  7 Packages

Total download size: 4.7 M
Installed size: 20 M
Is this ok [y/N]: y
...
  Running scriptlet: sigil-0.9.14-7.fc33.x86_64                             7/7 
  Verifying        : minizip-2.10.2-1.fc33.x86_64                           1/7 
  Verifying        : python3-regex-2020.11.13-1.fc33.x86_64                 2/7 
  Verifying        : FlightCrew-sigil-plugin-0.9.1-21.fc33.x86_64           3/7 
  Verifying        : python3-cssselect-0.9.2-16.fc33.noarch                 4/7 
  Verifying        : python3-cssutils-1.0.2-9.fc33.noarch                   5/7 
  Verifying        : sigil-0.9.14-7.fc33.x86_64                             6/7 
  Verifying        : zipios++-0.1.5.9-25.fc33.x86_64                        7/7 

Installed:
  FlightCrew-sigil-plugin-0.9.1-21.fc33.x86_64                                  
  minizip-2.10.2-1.fc33.x86_64                                                  
  python3-cssselect-0.9.2-16.fc33.noarch                                        
  python3-cssutils-1.0.2-9.fc33.noarch                                          
  python3-regex-2020.11.13-1.fc33.x86_64                                        
  sigil-0.9.14-7.fc33.x86_64                                                    
  zipios++-0.1.5.9-25.fc33.x86_64                                               

Complete!

Fedora 32 : Testing the Bookworm software.

The current version of Bookworm (v1.1.2) supports eBooks in the following file formats: EPUB, PDF, MOBI, FB2, CBR, CBZ.

First, I install this software with dnf tool:

[root@desk mythcat]# dnf install bookworm.x86_64 
...
Installed:
  bookworm-1.1.3-0.1.20200414git.c7c3643.fc32.x86_64                                          

Complete!

I tested with some old EPUB and PDF files and I'm not very happy with formatting text on the page.

HI tested with some old EPUB and PDF files and I'm not very happy with the formatting for certain texts on the page, like source code in programming.

Bookworm does one thing and does it well for this simple reader.

You can help this project on GitHub.