I implemented a simple markup language, “Mod”, that was similar to the “POD” language used to format the Perl man pages. There's a summary here:

https://github.com/mjdominus/mod

The main desideratum was that I should be able to use the same sources to generate TeX to send to the compositors, HTML for the web version, and a plain text version output. (In the end I sent the HTML version to the compositors instead.) Also I wanted inline tests, so that when the book said "function `foo` returns 7" I could have an inline test that would check that automatically. I added some features I needed that POD didn't have, like a code to generate an index entry. And I fixed some things I didn't like about POD.

Markdown hadn't been invented yet or I probably would have modified that instead of POD. Pandoc hadn't been invented yet or I probably would have used it.

But Mod didn't take long to make, and it worked well enough that it didn't get in the way while I wrote a book in it.

Per https://hop.perl.plover.com/book/ he used a modified version of the "POD" markup associated with Perl.