.EPUB

Introduction

EPUB is the open standard for ebooks. It was developed by the International Digital Publishing Forum, which merged into the W3C in 2017; EPUB 3.3 became a W3C Recommendation in 2023. An EPUB is essentially a small website in a ZIP file: the chapters are XHTML pages styled with CSS, with images, fonts and a table of contents, so the text reflows to fit any screen.

Structure

The ZIP archive must start with an uncompressed file named mimetype whose only content is application/epub+zip, so the type can be read from fixed bytes at the start of the file. META-INF/container.xml points to the package document, an .opf file that holds the book’s metadata, a manifest of every file and the spine, which sets the reading order. EPUB 3 books have a navigation document in XHTML; EPUB 2 books use an NCX file instead.

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