EPUB / MOBI formats

Definition

EPUB is a ZIP container of HTML, CSS, and metadata files, designed to adapt to any screen and font size. MOBI (and its proprietary successor KFX) comes from the Kindle ecosystem: Amazon now accepts EPUB upload but converts internally. A good digital file is not a shrunk PDF: it requires dedicated book design, a navigable table of contents, clean metadata, and EPUBcheck validation. In self-publishing, EPUB quality directly affects reader reviews and library catalog inclusion.

What it's for

Mastering these formats lets you publish on Kobo, Apple Books, Google Play, and Kindle without redoing everything per platform. EPUB 3 supports accessibility (semantic structure, alt-text), embedded fonts, and — in fixed layout — illustrated albums. For authors, understanding reflowable vs. fixed layout avoids delivering a graphically rich manuscript that is unreadable on e-readers. It is also the exchange format with traditional publishers' digital distributors.

Variants by genre

• Fantasy: long volumes in reflowable; maps in fixed layout or image inserts; glossaries at EPUB end with navigation anchors.
• Romance: standard reflowable; watch italic passages and drop caps that behave inconsistently across devices.
• Crime / thriller: short chapters benefit from granular TOC; evidence images as compressed inserts.
• SF: code blocks or log entries in <pre> or dedicated CSS classes; test on Kindle and Kobo.
• Literary: typographic sobriety; accessible EPUB (DAISY/Ace) increasingly required by libraries.

Common pitfalls

• Exporting directly from Word or Google Docs: dirty HTML, inline styles, EPUB rejected by platforms.
• Ignoring EPUBcheck: hundreds of silent errors until crash on a device.
• Neglecting accessibility (heading hierarchy, image alt-text): exclusion from institutional catalogs.
• Confusing PDF and EPUB: PDF is fixed; EPUB must reflow.
• Uploading obsolete MOBI when Amazon prefers EPUB source since 2022.
• Omitting metadata (ISBN, language, BISAC category): weakened discoverability in digital stores.

Examples

• Sigil — open-source EPUB editor for manual code control.
• Vellum, Atticus — multi-platform export pipelines for self-publishers.
• EPUBcheck — reference validator before any upload.
• Calibre — conversion and testing on simulated device profiles.
• IngramSpark / Draft2Digital — standardized EPUB distribution to retailers.

Writing tips

• Run the EPUB through EPUBCheck before any submission: an invalid EPUB is silently rejected by Apple Books and some distributors.
• Use 2 styles max (Body, Heading): EPUBs bloated with CSS break on old e-readers. Restraint is technical, not aesthetic.
• Test on 3 devices: e-ink reader, Kindle iOS app, Kindle Previewer desktop. A font that renders in Previewer can break on Kindle Paperwhite.
• Embed fonts sparingly: every font weighs into EPUB size and inflates Amazon's delivery fee. Stick to system fonts.

Learn more: Complete guide: migration & export (DOCX/EPUB/PDF) to submit your manuscript and Export DOCX, EPUB, PDF: how to prepare your manuscript for editors. See also the ebook entry for general digital-book vocabulary.

Revision checklist

• Does my EPUB pass EPUBcheck without blocking errors?
• Is the TOC navigable on Kindle, Kobo, and Apple Books?
• Have I tested at least three font sizes and two physical e-readers?
• Are images compressed without losing detail legibility?