Rather late to the party and I've already forgotten the initial email. Nevertheless, I'll give the program I most use: epub2txt.[0] It's not perfect, but compared to calibre's ebook-convert, and everything else I found in C in github or codeberg or gitlab, it's the best. A once-over with an editor capable of multiple selection and edition is the most I've had to do. Faulty output includes, say, only a single letter rather than a whole word capitalised or within '\e[...m' and '\e[0m'.
Protip; Run it with -w 0 to get 'natural' paragraphs. [0] https://github.com/kevinboone/epub2txt2
