Hi all,
I'm Stewart, a software developer living in Naarm/Melbourne.
I thought there might be some people here interested in a browser app I've created for authoring valid EPUB 3 books from plain text sources.
The app is called SEED.html
It's a little bit idiosyncratic (odd), and very new, and under active development.
You can use it on my web site, or download it to your own computer. It runs on all operating systems including iOS and Android though it's easier to use on a desktop.
https://stewarthaines.com/epub
Let me know how you go!
cheers, Stewart