I think the cover is important in book stores for that initial attraction. If I'm not familiar with the author, it's an intriguing title and cover that makes me pick up a book. However, it's the synopsis and blurbs on the back, and the first couple of paragraphs of the book that determine whether I buy it or not.
In digital marketplaces, it's harder to say. A strong book cover is still likely useful for attracting the eye, but again, it's the summary of content that will be the thing that sells it.