Self-published or traditionally published; good cover design is essential for any book. Period. Here is a fascinating article from the Indie Book Writers blog that shares the thoughts of some book designers about how the see their craft.
It provides a list of six simple things you can do to make your book stand out from the crowd and help sell ahead of the millions of other titles out there.
Now these tips are provided by experts in the field, so this is valuable information based on professional experience we are talking about.