Sigil is a multi-platform WYSIWYG ebook editor. It is designed to edit books in ePub format.
Features
Free and open source software under GPLv3
Multi-platform: runs on Windows, Linux and Mac
Full Unicode support: everything you see in Sigil is in UTF-16
Full EPUB spec support
WYSIWYG editing
Multiple Views: Book View, Code View and Split View
Metadata editor with full support for all possible metadata entries (more than 200) with full descriptions for each
Table Of Contents editor
Multi-level TOC support
Book View fully supports the display of any XHTML document possible under the OPS spec
SVG support
Basic XPGT support
Advanced automatic conversion of all imported documents to Unicode
Currently imports TXT, HTML and EPUB files; more will be added with time
Currently exports EPUB and SGF (Sigil native format); more will be added with time
Embedded HTML Tidy; all imported documents are thoroughly cleaned; changing views cleans the document so no matter how much you screw up your code, it will fix it (usually)
An actually usable user interface
Native C++ application
What’s new in version 2.3.1
fix to prevent double epub load warnings
fix to prevent load warnings without details from causing a crash
fix to prevent rename templates from using characters forbidden in file names and paths
fix to better restoreState when MainWindow is Maximized of FullScreen
fix lost External Editor Preference when browse cancelled (BeckyEbook)
fix for crashes related to serious epub load exceptions
Will sigil ever be compatible with the M1 processor?? I've tried multiple times this year (Nov 12, 2021 today) and it never works. Have a 2020 MacBook Air.
Just a note: This hasn't been a WYSIWYG editor in a while. The dev removed that portion of the app and split it off into a separate app called PageEdit (also open source).
I have used this app only once when Amazon was refusing my book in Calibre's mobi format and it saved my life. :-)
I opened with Sigil the epub generated by Calibre and generated the TOC (Tools/Toc/Generate at that time) then I opened the edited epub in Kindle Previewer or kindlegen and converted to mobi.
So 5 stars for this invaluable tool!