To succeed as a professional author, you need to write superb books, and never more than at present with the unprecedented level of competition in publishing today. (For example, when I sell a copy of a science fiction novels through Amazon, I have to compete against 169,000 other SF books, and beat every one of them.) But writing great stories is obvious; there are other things you need to do too.
I’ve been writing and publishing for a living since 2011, and if there’s one ‘other’ thing I’ve learned in that time, it’s to get the typography on your cover right for both your book and your brand. And if there’s another thing, it’s to get the right negative space on your cover. And then there’s the use of color. And having the image simple and strong enough to look good in a thumbnail.
Yes, covers sell books; there’s no getting away from that but they are also a fascinating topic in their own right. One of the most informative and fun places to think about covers is on M.L.S. Weech’s website where you get to vote on a series of cover contests to find the cover of the month. Here’s June’s winner being interviewed about his second win, for the excellent cover of ‘For Steam and Country’ by Jon Del Arroz.