Some thoughts in no particular order...
You could download the FREE app named "Calibre", which can open several different formats.
Then, try to read it using Calibre.
Calibre has become my preferred app for reading e-books.
What happens if you RIGHT CLICK on the icon for the book?
Do you get a contextual menu that shows you "Open With..."?
If you go there, does it give you the option to open the file using OTHER reading apps...?