This is a step by step guide shows how to install Sigil ePub ebook editor (v2.4.0 so far) by either using Flatpak package or building from the source tarball.
Sigil is a popular free and open-source ePub ebook editor that works in Windows, Linux, and macOS. However, it does not provide official packages for Linux.
While Sigil in Ubuntu system repository is always old, user can easily install the latest version via Flatpak package that runs in sandbox environment, or by compiling it from the source tarball.