I just discovered Foliate[0,1], which is a very small reader (252kb, deb) for .epub, .mobi, .azw, and .azw3 files. It's based Epub.js[2]. I was looking for a standalone epub reader (wanted to avoid the document management stuff) and stumbled across this gem.
It's performed very well on the epubs I've tossed at it.
Might I suggest you try PDF-XChange Editor?
https://www.tracker-software.com/product/downloads
This is an incredibly capable product, and free for both personal and commercial use. Some features are unlocked by licensing, but there's even an option to hide those if you have no need for them.