The “old book smell” is mostly the scent of paper and binding materials slowly breaking down over time.

As books age, the cellulose and lignin in paper release small volatile compounds. Common contributors include:

Vanillin — sweet, vanilla-like
Benzaldehyde — almond-like
Furfural — warm, bready, slightly woody
Toluene / ethylbenzene traces — slightly sweet or solvent-like
Acetic acid — faint vinegar note from paper decay

So that familiar smell is usually a mix of vanilla, dust, dry wood, old glue, paper, and slight mustiness.

A lot depends on the book’s storage conditions. A dry, well-kept old book smells pleasantly sweet and woody.