EPUB 3 Specification: See the World Wide Web Consortium (W3C) EPUB 3 page.
When you unzip an EPUB 3, you see one file and two folders:
File: mimetype
Folders: META-INF and OEBPS
The META-INF folder, which contains the book's metadata information, can contain one or more files. But it must contain a container.xml file. This file tells Apple Books how to process the rest of the EPUB package. See Container (container.xml).
The Open eBook Publication Structure (OEBPS) folder contains the book’s content, metadata, style, and table of contents. In the OEBPS folder:
The package document (.opf) includes all of the information about a book including metadata, manifest, and spine. See Package Document (.opf).
The navigation document (toc.xhtml) includes information about how to navigate a book and includes the table of contents, landmarks, and page-list. See Navigation Document.
The following sections break down the structure and describe the requirements.