I realized that the link I gave is not the newest version, and while investigating I got the impression that the book was created from a POD source.

Then I stumbled over this

https://sourceforge.net/p/pdl/pdl-book/ci/master/tree/

Please note the included scripts to build a PDF and even an all in one HTML.

See also the sub directories with a POD file per chapter.

Starting from there it should be easy to write a script to build ePUB from POD. You may even contribute it to the project.

Disclaimer: no guarantee whatsoever for the actuality of the links given. I'm not part of this community and I find the repository structure kind of confusing.

Update

Maybe also of interest App::Pod2Epub