HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.
calibre - Ebook manager, viewer & converter
Dr.Explain - Dr.Explain software is a help file authoring tool.
FBReader - FBReader is an e-book reader for various platforms. Features:
CHM Editor - Best solution to edit CHM files
Google Play Books - Google Play Books lets you search, preview, and buy millions of books using Google Book Search.