This looks like a source for a leanpub.com book (Book.txt gives it away, and the fact that this is all in a manuscript folder). I don’t think they have a command line tool to build epubs, but you should be able to get an epub from their web site if the author published it there

calibre [1] is also a pretty good open source tool, however there is a bit of work involved to convert html pages into epub.

[1] https://manual.calibre-ebook.com/faq.html#how-do-i-convert-a...

You can use pandoc if you don't mind a little bit of manual work:

https://gist.github.com/bmaupin/6e3649af73120fac2b6907169632...