To judge a book by its cover means to make a decision based only the outer appearance of a person or object or situation. It refers to the fact that we often think a book will not be a good read based on its tacky cover.
1. Five star hotels often look askance on a guest who enters in shabby clothes, but they should not judge a book by its cover; after all, that guest could be a rich person whose clothing was spoiled during his journey.
2. Although Sahil was not coherent in his speech, others in the firm realized that they should not judge a book by its cover as he was an ace in accounting.
3. The gold necklace which Mrs. Jain wore sparkled a good deal, so her friends thought it was real till she told them it was artificial and not to judge a book by its cover.