https://scienceswitch.com/2023/08/19/why-do-old-books-smell-so-good/

TL;DR:

An almond scent comes from benzaldehyde in the paper. Vanillin, the main compound in vanilla, is responsible for a sweet vanilla fragrance. Ethylbenzene, used in inks and paints, has a sweet plastic smell. 2-Ethyl hexanol, found in solvents and scents, smells lightly floral.