Inasmuch as there’s not, yet, an EPUB reader for Emacs, you can still set up Emacs to be able to open .epub files to see what’s inside them, since they are, after all, just ZIP files:
(setq auto-mode-alist
(append
(list
'("\\.epub$" . archive-mode))
auto-mode-alist))
(setq auto-coding-alist
(append
(list
'("\\.epub$" . no-conversion))
auto-coding-alist))