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:
WinCHM - Creating CHM file, Make CHM file, Web Help. WinCHM Pro - Help authoring software (CHM, WebHelp, DOC, PDF) . With this powerful tool you can not only create chm file from scratch, but also convert a set of existing HTML files to a chm file.
Google Play Books - Google Play Books lets you search, preview, and buy millions of books using Google Book Search.
Document360 - Self-service knowledge base software focused on SaaS Products & Enterprise Software Projects.