Sigil 1.4.1 [Update]

Sigil

Sigil is a free, open source, multi-platform ebook editor. It is designed to edit books in ePub format (both ePub 2 and ePub 3). It also provides features for advanced users, like direct XHTML, CSS and XPGT editing. You can use it to add any of the metadata entries supported by the EPUB specification and create a hierarchical Table of Contents.

Sigil key 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!

Sigil 1.4.0 new features:

Two different types of Multiple Language Spellchecking support added:
A user can now specify a Primary and a Secondary Language dictionary for real-time spellchecking (red squiggley underline).
The Spellcheck Dialog will now support spell checking as determined by xml:lang and lang attributes in any languages with dictionaries
Main UI icons now use scalable svg files to better serve higher res displays. All provided by BeckyEbook (ala BeckyDTP). A huge Thank You! to BeckyEbook
New brighter Sigil Application Icon is now being used.
Alternate Sigil main Application Icon now available as a build-time option
Sigil now provides 3 different Icon Themes for the user: Main, Fluent, and Material. Again all thanks goes to BeckyEbook for creating/collecting all of the icon themes!
Sigil will now support a custom user-determined Icon theme via a custom .rcc file.
pre-made icon themes for Legacy and Material themes in different base colors are now available with more user-contributed theme files to hopefully come. Again Thank BeckyEbook!
checkout https://github.com/Sigil-Ebook/IconThemes for more information and additional themes
New main toolbar icons are now available and improved icon groupings have been made to provide the user with a greater degree of toolbar customization. Users may need to drag and drop icon grouping and modify their selections of Toolbars to see some of these new options.
Improved support for IRI (international) hrefs. OPF newly added file manifest ids now converted to plain ascii to prevent issues with older readers

Bug Fixes:

Fix bugs in Marking for Index
Perform major URL en/decode refactoring to properly handle '#' (and other reserved chars) in file names
Translate "Table of Contents" using book language not ui langauge
Fix handling of links when using Add Existing on xhtml files with links to other xhtml files
Fix split code to better handle duplicate target ids that reside in different files
Bug fixes for the plugin interface from wrCisco (Thank you!)
Bug fixes for the plugin interface setguide from BeckyDTP (Thank you!)
Fix bug in OPF parsing when valid but non-traditional namespace prefixes are used
Fix rare off by one condition in clip number to clip enditor model conversion

Sigil 1.4.1 fixed bugs: