His desk is a mess, but don't judge a book by its cover - he does careful, detailed work.

don't 是 do not 的缩写形式，表示"不要、别"。 judge 的意思是"判断、评价"。 a 是不定冠词，表示"一个"。 book 的本义是"书"，这里用作比喻。 by 表示"通过……方式"。 its 是物主代词，指代前面提到的 book。 cover 指"封面"。 don't judge a book by its cover 是英语习语，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"的意思。