An EPUB is not a file per se; rather, it’s an archive of several files, like a ZIP or RAR archive. In point of fact, EPUB is ZIP. We’ll unzip an EPUB in a moment.
Inside the EPUB are all the files comprising the ebook, including the actual book text, a CSS file to define the presentation of the book text, any images and multimedia used in the ebook, the fonts used if you’ve chosen to embed some, and a couple of files containing a manifest and the book metadata such as title, author, publisher name, ISBN, and so on. Each of those files has a specific purpose, and each can be edited manually in a variety of applications, which is one of the many things you’ll do in this chapter. First, let’s get to those files.