GNOME Books is a simple application to access and organize your ebooks. It is meant to be a simple and elegant replacement for using a file manager to deal with ebooks. The app supports comic books and ePub books.
For my taste it's way too simple. All books "must" be in the ~/Document directory and it seems that there is no preference setting to change this. Opening a book directly with $gnome-books foo.epub won't work either.