That unmistakable aroma you get when you open a well-loved tome is a result of chemistry. As a book ages, the glue, paper, and ink begin to break down and release volatile compounds. A study in 2009 found that the chemical makeup of a book’s aroma consisted of hundreds of these compounds, giving it an organic mustiness with grassy notes. One main compound is lignin, which is present in all wood-based paper. Lignin is closely related to vanillin, which is what lends that faint, comforting smell of vanilla. No wonder curling up with an old book and a cup of tea sounds so inviting.