This English expression is very commonly used in both spoken and written English. It is used as a metaphor although it can also be used in its literal sense.
The expression is used to mean that you shouldn’t judge something based solely on its appearance, rather, you should look deeper and beyond first impressions before judging it. It can be used to refer to anything and is often used to refer to people and the fact that one should get to know someone before making judgements.
The expression can be dated back to the mid 19th century in the 1867 edition of the newspaper Piqua Democrat, which writes “Don’t judge a book by its cover, see a man by his cloth, as there is often a good deal of solid worth and superior skill underneath a jacket and yaller pants.” It is suggested that this phrase originally was used in spoken language when people visited bookshops and were immediately attracted to a book based on its appearance and as a result would often miss out on books which had less appealing covers but could be equally interesting inside. It was then extended to be used in a less literal sense but with the same overall meaning.
An example of the usage of this expression can be found in George Eliot’s novel The Mill on the Floss, in which the character Mr Tulliver used it in its literal sense when referring to a cover of a book and saying that Daniel Defoe’s The History of the Devil was “beautifully bound and one should not judge a book by its cover.”