A colorful color catches the eye. I learned that the book's design should replicate the text on the interior, thereby reflecting the image of your context or the reason you are writing the book. I hope that helps.
Writing a book is draining and stressful. The only thing that worked for me was getting up from the computer and returning to it after a couple of hours or the next day. In fact, my book addresses chronic stress and how it affects the body.