calibre - Ebook manager, viewer & converter
Sigil - Sigil is a multi-platform WYSIWYG ebook editor. It is designed to edit books in ePub format.
FBReader - FBReader is an e-book reader for various platforms. Features:
Vellum - Free beautiful hand-curated wallpapers for the iPhone
Google Play Books - Google Play Books lets you search, preview, and buy millions of books using Google Book Search.
Pressbooks - Create books: print and ebooks