I've previously preached that an author should have an image (certainly mentally but even better on paper or on a monitor) of the cover for an upcoming book. Seeing the cover image makes the book seem more "real" and may give the author an extra push to finish writing.
More importantly, I've found that the cover design, the title and the book can evolve simultaneously and interactively, influencing and improving each other.
Here are some versions of the cover and title of my newest book, Do As I Say, Not As I Did. This book was published on 6/1/14 but I initially conceived of it about two years earlier and the cover went through many changes -- some major and some subtle.
The book was on THREE Amazon bestseller lists the day after it was published. That's not really significant but it feels good to know that people are buying the book.