Generally, it is the chemical breakdown of compounds within paper that leads to the production of ‘old book smell’. Paper contains, amongst other chemicals, cellulose, and smaller amounts of lignin. Both of these originate from the trees the paper is made from; finer papers will contain less lignin than, for example, newsprint. In trees, lignin helps bind cellulose fibres together, keeping the wood stiff; it’s also responsible for paper yellowing with age, as oxidation reactions cause it to break down into acids, which then help break down cellulose.

via What Causes the Smell of New & Old Books? | Compound Interest.