DavidP wrote:
https://www.google.ca/search?source=hp&ei=mYS_XbH4CtLk_Aa-tJSQDA&q=epub+reader+linux&oq=epub+reader&gs_l=psy-ab.1.8.0l10.3379.9610..15029...0.0..0.81.825.11......0....1..gws-wiz.....0..0i131.OQOBJyaWPmY#spf=1572832423686
OK. Four years later, this answer is kind of funny but, still not useful. I actually had searched and tried a few different programs. What I eventually figured out was that the file associations could be messed up and instead of opening an epub file with the Calibre Ebook Reader, by default, my file manager was opening it with the editor! That was very frustrating for me but, once I got that sorted, I've been happily buying and reading stuff from Seaforth and I like the Calibre Project.
Hopefully someone will find this helpful, in the future.