calibre - Ebook manager, viewer & converter
Filza File Manager - Explore files and folders with list view or grid view. View / edit files with powerful viewers.
FBReader - FBReader is an e-book reader for various platforms. Features:
Documents by Readdle - A central hub for all your files.
Google Play Books - Google Play Books lets you search, preview, and buy millions of books using Google Book Search.
FileApp - FileApp is an iPhone and iPad File System and Document Viewer.