According to professional book editors professional designer brings more to the table than just Photoshop skills. Book cover design is a delicate balance of images, text, and information. To sell your book effectively, you'll need someone who understands how each of these elements interacts with the others.
A book jacket designer has a good understanding of how readers react to layout and typography. They are aware of current trends and can predict how your cover will fare in the marketplace. Most importantly, they'll know how to use your body to send the right message