calibre - Ebook manager, viewer & converter
HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.
FBReader - FBReader is an e-book reader for various platforms. Features:
HelpSmith - HelpSmith is a help authoring tool to create CHM HTML help files, manuals, and PDF documents.
Google Play Books - Google Play Books lets you search, preview, and buy millions of books using Google Book Search.
Adobe RoboHelp - Adobe RoboHelp empowers organizations to deliver engaging help and policy content across mobile devices, formats, and platforms.