The Mass Explained App for iPad looks like a pretty awesome app ("coming soon") from Agnus, LLC. I haven't seen the app in action (just screenshots), but even if it's just a bunch of static pages like an interactive book, it looks like the content is high-quality:
I really hope the app function as good as it looks. If so, I think this would be a good template for other similar apps explaining sacraments, sacramentals, the Rosary, prayer, saints, the Vatican, etc.
Basically take encyclopedic knowledge of a Catholic subject, and marry that with some great illustrations, images, and 3D models, and make a very nice interactive app or iBook. I would buy the whole series!