The idiom "judging a book by its cover" means taking the face value or apparent value of something as the final evaluation. In fact, books with good covers may not have good contents at all, and vice versa. Hence, we must not judge a book by its cover, or judge a person or thing or situation primarily on their face values.
Rachel made a mistake in thinking her fiancée as a philander since he had an easy-going nature; she had the habit of judging a book by its cover.
Upon seeing a little child in tattered clothes, little Emma looked at him with disgust. On seeing this, Emma's mother told her never to judge a book by its cover.