Calibre on Linux

Good news: Calibre runs on Linux. It's the same app you use on Windows, and TuxPick installs it for you from Ubuntu's own software archive.

Calibre

Also on Windows

Manages your ebook library and converts between ebook formats.

Installs from Ubuntu's own software archive.

Press the install button above. TuxPick opens with the app already ticked. Add anything else you need.
Download the installer file.
Open a terminal with Ctrl Alt T, run the one line TuxPick shows you, and type your password when asked.

It works on Ubuntu 22.04 and 24.04, and on systems built on them like Zorin OS and Pop!_OS.