Don't delete your existing library without backing up first, Fireland. Remember, you have the backup feature in your Marvin. At the very least, before deleting anything, perform a full backup of your Marvin. It will back up
everything for you: all your books, all your settings at the touch of a button. See here:
http://blog.marvinapp.com/post/67661991764
But I'm sure this can be resolved without you deleting anything. App Store is definitely acting up right now. For example, it took 6 attempts (!) for me to update my original Marvin version. The first 5 attempts, App Store just sent me an error message, "Unable to download the app at this moment."
Please search through your iPad using the spotlight search. Maybe you
will find the two Marvins!

iOS may have put the new Marvin's icon in an empty space on one of your secondary iPad home screens, where you can't readily see it.
DuskyRose, the new Marvin version is 2.4 (on all devices).
PS: ninja'd by Wulf.