One should never judge a book by its cover, as things aren't always what they seem.

judge 是动词，意思是"评判、判断"。 a 是不定冠词，表示"一个"。 book 本义是"书"，这里用作比喻。 by 是介词，表示"通过...方式"。 its 是物主代词，指代前面提到的"书"的。 cover 本义是"封面"，这里与book搭配使用。 judge a book by its cover 是英语习语，字面意思是"通过封面来评判一本书"，实际含义是"以貌取人"或"凭外表判断事物"。