Although his desk is messy, you should not judge a book by its cover; his work is actually very meticulous.

not 是否定词，表示"不"或"不要"。 judge 是动词，意思是"判断、评价"。 a 是不定冠词，表示"一个"或"一种"。 book 是名词，意思是"书"。 by 是介词，表示"通过"或"依据"。 its 是物主代词，指代前面提到的"书"的，意思是"它的"。 cover 是名词，意思是"封面"。 not judge a book by its cover 是一个英语习语，字面意思是"不要通过封面来判断一本书"，引申为"不要以貌取人"或"不要仅凭外表判断人或事物"。