Have the EPUB somewhere on your computer where you can navigate to it. This may require downloading it from an external source.
In iTunes, select the file menu, File > Add to Library
This then opens up the Add To Library navigation box:
Select and Choose the Title you wish to add.
Now be sure the Books > Sync Books setting for your iPhone / iPad is either set to All Books or Selected Books with the title you wish to have on that device selected.
Now, Sync your iPhone / iPad