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Sigil eBook editor

What is Sigil?

Sigil is a multi-platform EPUB ebook editor (think Word or LibreOffice but specific for EPUB ebooks) with the following features:

https://sigil-ebook.com/about/

Free and open source software under GPLv3
Multi-platform: runs on Windows, Mac, and Linux (note Linux only receives limited support)
Full UTF-16 support
EPUB 2 spec support with limited EPUB 3 support
Multiple Views: Book View, Code View and Preview View
WYSIWYG editing in Book View
Complete control over directly editing EPUB syntax in Code View
Table of Contents generator with multi-level heading support
Metadata editor
User interface translated into many languages
Spell checking with default and user configurable dictionaries
Full Regular Expression (PCRE) support for Find & Replace
Supports import of EPUB and HTML files, images, and style sheets,
Documents can be validated for EPUB compliance with the integrated
All imported files have their formatting corrected, and your editing can be optionally cleaned
Plugins!