Cedar Sanderson at Mad Genius Club has a an article on book covers http://madgeniusclub.com/2016/01/09/comprehensive-cover-art-and-design/
Some good advice in particular:
First and most important: before you start designing a cover, creating art intended for book covers, or even thinking about a book cover, you need to look at book covers. A lot of them. Specific book covers to your genre is even better, as there are subtle cues you need to know and recognize, even if you aren’t doing your own covers.
Other aspects I only part agree with. There is an emphasis that what works as marketing trumps other considerations and that are some genre boundaries that should be crossed that are a bit Nutty-Nuggetsy.