Do I have the right to display a book cover in my mobile application assuming it's provided by Google Books and I give Google credits?
(I don't earn money, don't display ads and don't copy the image on my server)
It depends on the context, purpose and use. It might be fair use or allowed similar exceptions in other countries or could be a copyright infringement.
For example, using book covers images in a blog entry doing a critical analysis of those books covers is OK under right to quote (as covered by Spanish law) and it would be OK under fair use. Displaying book covers in relation to news coverage (of news related to the books) or to advertise the books seems to be OK under US law.
Anyway, some other uses aren't covered. For example, use of covers of Pokémon books as a substitute for cards in a game in your app wouldn't be allowed.
Therefore, to give a useful answer we need to know more about how do you plan to use those images.