An epub file is just a zip file with plain text chapters + images + table of contents inside laid down in a structured (read: standardized) manner. You can literally extract an epub file to a folder with a zip tool (like WinRAR if you use windows) and use notepad to read the book if you want.

And, even if it weren’t a fancy zip-based file format, but a binary format, the specification is out there and open freely.