Don’t judge a book by its cover

An adage that means one can never rely on appearances alone, which appears to be of American origin from the late 1920s and rapidly gained acceptance in Britain. The concept, however, is probably much older. for example, in George Eliot's Mill on the Floss, published in 1860, the characters discuss the merits of a book, how good it is etc. and one of them says, "You cannot tell from the outside." Is this a possible first citation of the adage or the concept? The question is debatable.