Okular (with all due respect to it, Evince is in the similar boat) is an excellent PDF reader, but not so great EPUB one. There is a whole lot of great EPUB readers (EPUB - Wikipedia, but that’s just very brief overview, there are many more) starting with the simplest one like the terminal epy (I have just made extensive improvements to it), through various browser addons (e.g., EPUBReader) to stand-alone apps.