Sigil Portable is an ePub ebook editor with support for both WYSIWYG word processor style and direct code editing. It's packaged in PortableApps.com Format so it can easily integrate with the PortableApps.com Platform. And it's open source and completely free.

Sigil is a multi-platform EPUB ebook editor with the following features.
Sigil provides advanced automatic conversion features for all imported documents to the Unicode text encoding.
Sigil also offers a metadata editor with extended support for all types of metadata entries (over 200) that enable you to insert a full description for each.

Features
* Online Sigil User's Guide, and Wiki documentation
* Full UTF-16 support
* Full EPUB 2 spec 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 with full support for all possible metadata entries (more than 200) with full descriptions for each
* 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 FlightCrew EPUB validator
* Embedded HTML Tidy: all imported files have their formatting corrected, and your editing can be optionally cleaned

Sigil Portable is packaged in a PortableApps.com Installer so it will automatically detect an existing PortableApps.com installation when your drive is plugged in. It supports upgrades by installing right over an existing copy, preserving all settings. And it's in PortableApps.com Format, so it automatically works with the PortableApps.com Platform including the Menu and Backup Utility.

Sigil 0.8.4 - 2015.02.02
With only one day after the 0.8.3 release 0.8.4 is being released. My deepest apologies because this means there is a bad bug. Admittedly it will only impact a very small amount of users but it was big enough to warrant a new release.
Unfortunately during refactoring of “remove all plugins” code the actual remove code was moved to the wrong place. With 0.8.3 if you say no it will still remove all plugins. This has been corrected and is the only change in 0.8.4.
- Fix for removeall plugins bug where all plugins would remove all plugins regardless if the user clicked no instead of yes.
Sigil 0.8.3 - 2015.01.31
- 10.9.5 is now the minimum OS X version required. Only 10.10.1 has been tested.
- -DCODE_SIGN_ID=XYZ option has been added to auto sign the app bundle on OS X.
- Validation result plugins will auto close the Plugin Runner Dialog since the dialog has no real info and openes the validation pane with the results anyway.
- New Plugin Preferences Support added
- Plugins Manager will now remember last folder used
- Support for building both Linux .deb and .rpm packages
- Lots of Linux build support, compilation, installation, and documentation improvements
- Qt 5.4 is now a minimum requirement.
- Build changes to support update to latest Qt 5.4.0 and to use latest C++11 compiler
- Numerous bugs fixed or worked around including:
* Make Invocation of CSS W3C Online Validator more reliable
* Fix to make Preview Window update after BookView to prevent memory corruption and crashes
* Workaround Qt bug (font cache assertion) when using clearMemoryCaches()
* Properly handle loadFinsihed Signal in BookViewPreview to prevent clashes between BookView and Preview
* Fix for Sigil memory use after it was freed bug
* Workaround to the extent possible Qt bug with memory use after free using QWebInspectors
* Workaround to the extent possible Qt bug with memory use after free when closing MainWindows
* Bug fix to prevent crashes in KeyboardsShortcuts when accessing Preferences with multiple MainWindows