Sigil is an editor for the EPUB format. It is designed for WYSIWYG
editing of EPUB files and for converting other formats to EPUB. It
also provides support for direct XHTML, CSS and XPGT editing. You can
use it to add any of the metadata entries supported by the EPUB
specification and create a hierarchical Table of Contents.
There is no official package available for openSUSE Leap 15.5