Most people treat EPUB as a black box. You throw a manuscript at some tool and hope for the best. But if you actually understand what's inside an EPUB, you gain total control over formatting, layout, and compatibility. So let's crack one open.

What Is an EPUB, Really?

An EPUB file is a ZIP archive with a .epub extension. That's it. Rename any .epub to .zip, extract it, and you'll find a structured set of files inside. The EPUB 3 spec (maintained by the W3C) defines exactly what goes where.

Here's the typical structure:

my-book.epub/
  mimetype
  META-INF/
    container.xml
  OEBPS/
    content.opf
    toc.xhtml
    chapter-01.xhtml
    chapter-02.xhtml
    styles/
      main.css
    images/
      cover.jpg

The mimetype File

This one's dead simple. It must be the first file in the ZIP, stored uncompressed, and contain exactly:

application/epub+zip

No newline at the end. No extra whitespace. Readers check this first to confirm they're dealing with an EPUB.

container.xml

Inside META-INF/, this file tells readers where to find the main package document:

<?xml version="1.0" encoding="UTF-8"?>
<container version="1.0" xmlns="urn:oasis:names:tc:opendocument:xmlns:container">
  <rootfiles>
    <rootfile full-path="OEBPS/content.opf" media-type="application/oebps-package+xml"/>
  </rootfiles>
</container>

The OPF Package Document

content.opf is the heart of your EPUB. It has three sections: metadata, manifest, and spine. Metadata includes your book title, author, language, and a unique identifier. Manifest lists every single file in the publication — chapters, images, stylesheets, the nav document. Spine defines the reading order.

<package xmlns="http://www.idpf.org/2007/opf" version="3.0" unique-identifier="uid">
  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/">
    <dc:title>My Book Title</dc:title>
    <dc:creator>Your Name</dc:creator>
    <dc:language>en</dc:language>
    <dc:identifier id="uid">urn:uuid:12345-abcde</dc:identifier>
  </metadata>
  <manifest>
    <item id="ch01" href="chapter-01.xhtml" media-type="application/xhtml+xml"/>
    <item id="ch02" href="chapter-02.xhtml" media-type="application/xhtml+xml"/>
    <item id="css" href="styles/main.css" media-type="text/css"/>
    <item id="nav" href="toc.xhtml" media-type="application/xhtml+xml" properties="nav"/>
  </manifest>
  <spine>
    <itemref idref="ch01"/>
    <itemref idref="ch02"/>
  </spine>
</package>

Writing XHTML Chapters

Each chapter is an XHTML file. Not HTML5 — XHTML, meaning it must be well-formed XML. Self-closing tags, quoted attributes, lowercase elements. Keep your markup clean and semantic.

Use

,

through

,

, and

. Avoid

soup. The cleaner your markup, the better it renders across Kobo, Kindle (after conversion), Apple Books, and other readers.

Styling with CSS

EPUB supports a solid subset of CSS. You can control fonts, margins, line height, and colors. Where things get tricky: each reader app applies its own user-agent stylesheet, and users can override your choices (font size, typeface, background color).

Design defensively. Use relative units (em, %) instead of px. Don't assume a specific screen width. Test on multiple readers.

Validation with EPUBCheck

Before distributing your EPUB, run it through EPUBCheck. It catches malformed XML, missing manifest entries, invalid metadata, and accessibility issues. Most ebook stores (including Apple Books and Kobo) reject files that fail validation.

The Faster Route

Building an EPUB by hand is educational, but for day-to-day work you probably want a converter. If you have a Word document, PDF, or plain text file, you can convert it directly.

Convert files to EPUB with MyPDF — upload your document, choose EPUB as the output, and download a properly structured file in seconds.

Related Tools

• Convert to EPUB — Transform documents into valid EPUB files
• EPUB to PDF — Convert ebooks to PDF for printing or archiving
• PDF to Word — Extract content from PDFs before creating an EPUB

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