Sigil is a multi-platform EPUB ebook editor with the following features:

 * Free and open source software under GPLv3
 * Multi-platform
 * 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!