iOS 12.1 Files.app will open Google Drive hosted ePub files directly in Books.app (formerly iBooks). I don’t know how new this is, but tapping on the same file in Google’s Drive.app gives an “unsupported file type” error. (You can still copy it to Books, it’s just awkward.) I’d long used Drive.app to open my ePubs, just happened to try Files.app today.
Books UI doesn’t scale well to significant number of ePubs, storing them in Google Drive or iCloud Drive works much better . I treat iBooks as a temporary store, periodically I clean it out.
 Also iOS 12 Books.app won’t sync with Sierra iBooks, so those of us who are putting off painful updates have another reason to store in the file system. Really, though, it’s just way better than using iBooks storage. I’m a bit disappointed Apple hasn’t fully integrated iBook storage with iCloud Files, but this is nice.