EPUB format
EPUB is the open, standard file format for ebooks. Its specification is maintained by the W3C, the organization behind web standards, and the current major version is EPUB 3. An EPUB file is a ZIP archive that holds the book’s text as XHTML, its styling as CSS, its images, a package file with the metadata (title, author, language, identifiers) and reading order, and a navigation document that reading apps turn into the table of contents.
There are two kinds:
- Reflowable EPUBs let text adjust to the screen and the reader’s settings. They suit novels and most nonfiction.
- Fixed-layout EPUBs keep every page exactly as designed. They suit picture books, comics and heavily designed cookbooks.
Why it matters for indie authors
EPUB is the format most ebook stores work with, including Apple Books, Kobo, Barnes & Noble and Google Play Books. Amazon KDP accepts EPUB as well and converts it into Kindle’s own formats for delivery; it no longer accepts MOBI files. One well-made EPUB can therefore serve almost every store you publish in.
Example
An author finishes her novel and exports an EPUB. Before uploading, she runs it through EPUBCheck, the free, open-source validator, to catch broken links and missing files. Then she opens it in Kindle Previewer and in an EPUB reading app to check chapter headings, scene breaks, images and the table of contents. The same file goes to KDP and to the other stores she sells in.
In AIBookWriter, the Format & export step produces an EPUB alongside a print-ready PDF, DOCX and Markdown, with preflight checks before you export.
Common mistakes
- Skipping validation. Stores can reject files with errors that EPUBCheck would have caught.
- Hard-coded formatting. Fixed font sizes, manual line breaks and spaces used as indents fight the reader’s settings.
- Missing navigation. Readers expect a table of contents that links to every chapter.
- Oversized images. They make files heavy, and under KDP’s 70% royalty option, larger files carry higher delivery costs.
- Fixed layout for a text-heavy book. Readers can’t resize the text, which makes it hard to read on a phone.