To realise the potential of a manuscript, cover art must be clear and genre appropriate. From a designer’s perspective, writing is simply a process whereby marks reproduced on a two-dimensional surface miraculously trigger images and emotions in the mind of the viewer. It necessarily follows that the way these marks present to the eye is of crucial importance. The very least you should demand of a cover design is that it’s readable, eye-catching, and genre specific.
Regardless of the virtue of your book's content, potential readers are unlikely to be attracted unless its visuals meet these basic criteria. And once you get your design established, maximise its visibility. Let it flow into other branding opportunities, such as social media campaigns and promotional items, like bookmarks and posters.