Sigil (64-bit) is a multi-platform WYSIWYG ebook editor. It is designed to edit books in ePub format. It is free and open source software under GPLv3, It provide full Unicode support: everything you see in Sigil is in UTF-16 and full EPUB spec support, WYSIWYG editing, support Multiple Views: Book View, Code View and Split View, include Metadata editor with full support for all possible metadata entries (more than 200) with full descriptions for each, table Of Contents editor, perform advanced automatic conversion of all imported documents to Unicode.
Sigil is simple, fully EPUB and Unicode compliant, and is fun to use. You can edit/author in Sigil and then use Calibre to convert to different formats.
Cons
It looks like Ver 7+ is an x86 program and that the Version 6 - 64 bit Sigil has not been updated to a Ver 7+???? I am using the x86 Ver 7.1 on a Win 8 Desktop with no problem and would suggest going with the latest Version.
Summary
All that I had hoped it might be...
LEG21
Pros
Comparatively easy to learn and use
Cons
Some editing capabilities are either not possible with that kind of HTML files, or were not available in the software.
Summary
Sigil editing was largely intutitive for users of other editing software, was documented with the help files, and even explained something of the nature of ePub files and their relationships--a critical need for anyone new to ePub documents.