Probably purely true for some books, but as someone who buys thousands of books a year, my impression is they are very likely to reveal who they think their readers will be (hence a lot of covers say "stay away" to me), and just occasionally they can show a startling streak of originality. E.g. the board designs (there may be no dustjacket) on Dave Eggers' books are uniquely artistic in my opinion, and in this case since he has been seriously into graphics, I don't think it's any accident. You might think "Maybe this book is written by a bold and original person" and IMHO you'd be right. Also, the cover design of The Curious Incident of the Dog in the Night-Time by Mark Haddon kind of sent a message on my wavelength and it was not misleading (for me).