You can use calibre software for viewing .epub documents.

To install calibre from terminal:

sudo apt-get install calibre

Or click the icon below.

calibre

If you don't want to launch the full calibre client just to view your ebook files you can add a .desktop launcher to calibre's inbuilt ebook viewer:

1. Create a new .desktop file in ~/.local/share/applications:

   gedit ~/.local/share/applications/calibre-ebook-viewer.desktop
   

2. Copy and paste the following passage into the file:

   #!/usr/bin/env xdg-open
   
   [Desktop Entry]
   Version=1.0
   Type=Application
   Terminal=false
   Icon=calibre
   Exec=ebook-viewer
   Name=Calibre Ebook Viewer
   Comment=Display .epub files and other ebook formats
   MimeType=application/x-mobipocket-ebook;application/epub+zip;
   StartupWMClass=calibre-ebook-viewer
   

3. Save the file. You should now be able to launch Calibre's inbuilt ebook viewer both from the dash and through the right click "Open with" menu.