Sigil

Sigil is a free, open-source EPUB editor that lets you open an ebook and fix it directly at the source-code level — the HTML, CSS, and metadata that make up the file. It's not a converter that takes in Word documents; instead it's where you go after a rough EPUB comes out of Calibre or Pandoc and needs its table of contents, semantic markup, or metadata cleaned up before store submission. For authors and formatters who want precise control over the final file, nothing else in the free tier matches it.

Why Authors Use It#

• Best for: Authors and formatters doing hands-on EPUB cleanup after an initial DOCX-to-EPUB conversion elsewhere
• Direct source editing: Edit EPUB 2 and EPUB 3 HTML, CSS, and XML with both a code view and a WYSIWYG-style book view
• Built-in TOC and metadata tools: Generate and repair the table of contents, edit the OPF metadata, and manage the spine and manifest
• Validation built in: Integrates the EPUBCheck validator so you can confirm the file is store-ready before uploading
• Free and cross-platform: GPL-licensed, actively maintained, and available on Windows, Mac, and Linux

Limitations#

• No direct DOCX import: Manuscripts must first be converted to EPUB (via Calibre or Pandoc); Sigil polishes, it doesn't originate
• Technical learning curve: Getting real value out of it means working with HTML and CSS
• Not a layout designer: For themed, designed interiors, use a formatter like Vellum or Atticus

Alternatives#

• Calibre (free) — pairs naturally with Sigil for the initial conversion, and has its own lighter-weight EPUB editor
• Pandoc (free) — for producing the clean starting EPUB that you then refine in Sigil