So I just downloaded it and I'm curious, since i'm at work and don't want to play all day, does anyone know what folder to add my books to that I already own?
Android > Data > com.google.android.apps.books > files > volumes
I have no idea if the app will open these other ebooks you speak of, but it seems that if it did, they would make it a little easier to locate the destination. Please let us know if it works for you.