Book Covers are To Be Judged

"Don't judge a book by its cover" is a methapor that means we shouldn't judge a person, or something, solely on appearance or first impression. However, the very purpose of a book cover is to judge the book - it's marketing tool to hopefully intice people to buy the book. 

If we weren't supposed to "judge a book by its cover" then why does it have pictures, short summary, "New york time best seller" stickers ... or anything else meant to get the book in peole's hands?

The cover of a book is, by design, meant to be judged. We also tend to judge people on first impression and, if its negative, we must find it in our heart to give the person the benefit of the doubt. Maybe he was having a bad day, maybe he can't afford a nice suit, maybe he's anxious... let's give people a chance to redeem themselves.