Sigil

Sigil is a free, open source ebook editor that runs on Windows, macOS, and Linux. It creates and edits books in EPUB format, which is the standard used by most ebook readers and apps such as Apple Books, Kobo, and Adobe Digital Editions. Both older EPUB 2 and newer EPUB 3 files are supported.

EPUB files are built from web-style content, so Sigil gives you direct access to the HTML and CSS that make up each chapter and its styling. You can also edit a book's metadata such as title, author, and publication date, generate a table of contents automatically, and run reports to spot issues across the whole file. A built-in preview panel shows how the book will look to a reader, and a checkpoint comparison tool lets you see what changed between saved versions.

Sigil includes a plugin system driven by Python 3, which allows optional extensions for tasks like validating EPUB structure, checking grammar, converting file formats, and more. A community-maintained index of available plugins is hosted by the MobileRead forums. A companion application called PageEdit offers a visual HTML editor for those who prefer to write or arrange content without editing code directly.

On Windows, Sigil ships as a standard installer and is also available through package managers like winget and Chocolatey. On Linux, the developers provide an AppImage build for systems whose package repositories carry an older version. Mac builds require assembling several components before compiling, and the repository includes step-by-step documentation for each platform. The program is licensed under the GPLv3.