每本小人书都用牛皮纸加了层封皮】 every story book was covered with a layer of kraft paper. 层 means layer here, 两层纸 means "two layers of paper", likewise "A layer of dust" means 一层灰，stretch your imagination and you'll find it is not only used as a measure word for buildings.

Thank you for your reply :) Actually I know "measure word" in Chinese, but in that Chinese sentence there isn't any relative between the meaning of sentence and 层, because as I know 层only use for building, am I right?

It is a really good question and please read patiently. If you couldn't understand, I would like to explain for you. Actually, it is an obvious difference between Chinese and English. In Chinese, the words like 个，只，支，头 and also as you mentioned above 层. They are all called count word/classifier （量词 in Chinese） For example, in English I can just say 'three people', 'two ducks', 'a pen', 'three cows' and ' a cover of this book' directly. However in Chinese, when you are trying to describe these things, you should always with your count words in your sentence. The translations of examples are below. 三个人， 两只鸭子，一支笔，三头牛，一层封面 Now, do you see the differences? I hope you can fully understand and I 'd like to explain for you if you 've got further questions.