Check out Sigil. It's a multi-platform EPUB ebook editor with the following features:

 - Online Sigil User's Guide, FAQ, and Wiki documentation
 - Free and open source software under GPLv3 
 - Multi-platform: runs on Windows, Linux and Mac
 - Full UTF-8 support
 - Full EPUB 2 spec support
 - Multiple Views: Book View, Code View and Split View
 - WYSIWYG editing in Book View, supporting all XHTML documents under the OPS specification
 - Complete control over directly editing EPUB syntax in Code View
 - Table of Contents generator with multi-level heading support
 - Metadata editor with full support for all possible metadata entries (more than 200) with   full descriptions for each
 - User interface translated into 15 languages
 - Spell checking with default and user configurable dictionaries
 - Full Regular Expression (PCRE) support for Find & Replace
 - SVG support and basic XPGT support
 - Supports import of EPUB and HTML files, images, style sheets, and fonts
 - Documents can be validated for EPUB compliance with the integrated FlightCrew EPUB validator
 - Embedded HTML Tidy: all imported files have their formatting corrected, and your editing can be optionally cleaned
 - Native C++ application