Initially, my approach to designing a book cover is always literally tied to the title. My mind, unfortunately, just works that way, but I don't necessarily see it as a bad thing. To me, it's just getting those ideas out of my system. Then, I begin to really think outside of the box. Knowing the content definitely helps with ideas of where I can go. Sketching then comes into play, and the result is a bunch different scenarios that could work. Most of the time, I just combine different elements of different sketches until I get the idea that works best.
Researching cool book covers is always interesting to me because I think about what could have possibly motivated the designer to settle with their ideas. I know in my case, I would probably never initially come up with a crazy, conceptual cover. It would definitely take time to mold my ideas until I get what I think works best.