I was looking around for a truly minimalist CLI epub reader. Meh. Didnt find anything that worked that well. But did find what has to be absolutely most basic way of reading epub. Open terminal and use unzip command, it can unzip epub without renaming it. You will get several html files which contain the text, ignore any other files. Oddly Firefox refuses to open them. But Seamonkey can and is maybe most pleasant way to view them, so can little html viewer called Surfer that BarryK included in EasyOS. No menu entry, you have to call it from CLI. LibreWriter can. So can Geany and Leafpad though they show all the html tags so makes it unpleasant to try and read. Oh well I thought it was interesting.
Gotta say my favorite ebook reader is actually a free Android only one called Librera PRO. Its not minimalist but its pleasant to use. Honestly lot ereaders arent that pleasant. Usable but not pleasant.