ReLaXed really focuses on PDFs right now (to keep the initial focus small) but it produces an HTML file as a byproduct. From what I understand there is not far from an HTML file to an Epub.

This being said, the primary goal of this library is to enable to make documents with complex or fancy layouts. Epubs generally have a simple structure (chapter/section/paragraph) and can be written using for instance Markdown:

https://pandoc.org/epub.html