What is the smell of books in the library?

The smell of old books is caused by the decomposition of compounds in paper. Paper is made of trees, the main components are cellulose and a small amount of lignin. Lignin helps long-chain cellulose to gather together, thus making trees stand upright. Over time, it is decomposed into acid by oxidation, which promotes the decomposition of cellulose, thus producing various volatile compounds, which is the source of the smell of old books.