You don't "judge" a book by its cover; you use the cover as additional evidence to more accurately predict what's in the book. Knowing what the publisher wants you to assume about the book is preferable to not knowing.
(Except when it's a novel and the text on the back cover spoilers events from the middle of the book or later which I would have preferred to not read until the right time.)
Another monthly installment of the rationality quotes thread. The usual rules apply: