The less you offer, the more readers are forced to bring the world to life with their own visual imaginings. I personally hate an illustration of a character on a jacket of a book. I never want to have someone show me what the character really looks like - or what some artist has decided the character really looks like - because it always looks wrong to me. I realize that I prefer to kind of meet the text halfway and offer a lot of visual collaborations from my own imaginative response to the sentences.