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

• 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!