One thought: Book covers are made to sell books, so, though they may reveal something about the contents of the book, you can't judge how GOOD a book will be by it's cover. This works for many things...
It means you don't just look at something (or someone)and judge it (or him) right away. Ex. You see a book you should maybe do some research or read a little of it. Don't just look at the cover and...