I struggle with covers. So I'm hesitant to offer advice. But J. M. Ney-Grimm has done several very good articles on covers on her blog. The following link isn't the article I remember reading, but I think it still shows a lot of what can be done with design. Her first article gave interesting tips. For example the words should fill the page. If you look at the examples above, Brown fills the cover from side to side for Inferno. Also with Inferno, the initial "I" in Inferno appears to be larger than the rest of the type. Her advice was to dress the words up. (My description. Not hers. Hers was much better put.) Anyway, here is, at least, the link to her most recent posts on covers. I hope it helps. She really, I think, has a flair for them.
http://jmney-grimm.com/category/behi...over-concepts/