That's another way in which iOS 7 botched what used to work perfectly fine in iOS 6. In iOS 6, if Marvin was your preferred app for opening EPUB files, it would be offered by default as the first app to open a new EPUB file. But in hideous iOS 7, it's always iBooks that gets offered as the default EPUB app, much like in ancient iOS versions like iOS 4. So you need to manually look for Marvin's icon every time, wasting time.
It's reallly an embarrassement for Apple to fail to offer "default app" setups on iOS, not just for EPUB files, but also the default browser, mail app, calendar app, etc. One locked-down hell.
