Don't Judge a Book by Its Cover

The underlying idea is ancient, expressed by the Roman poet Juvenal around the 1st or 2nd century AD, warning against judging by appearances alone. The specific book-and-cover imagery developed much later in English: George Eliot's 1860 novel The Mill on the Floss references a handsomely bound book in a related sense, and a version of the modern proverb appeared as a newspaper headline by 1849. The exact wording used today is traced to a 1946 American mystery novel, Murder in the Glass Room, which used the phrase "you can never tell a book by its cover."