Why do old paperback books smell sweet?
As cheap paper ages, it breaks down into fragrant compounds that smell like vanilla, almond, and grass.
The paper is quietly falling apart
Most 20th-century paperbacks were printed on cheap wood-pulp paper that still contains lignin, the same stiff polymer that holds trees upright. Lignin is unstable. Over decades it reacts with light, heat, and the acids left over from manufacturing, and it slowly comes undone.
That browning and brittleness is the visible half of the story. The smell is the invisible half: the broken-down molecules are small and light enough to drift off the page and into your nose.
Decay smells like dessert
When lignin degrades, one of the compounds it releases is vanillin, the exact molecule that gives vanilla its scent. Other reactions throw off benzaldehyde (almond), furfural (warm and bready), and a handful of sweetish hydrocarbons. The 'old book' bouquet is really a slow chemical bake-off.
Why old books, not new ones
Archival paper made with low acid and little lignin ages far more slowly, which is part of why a 1970s pulp thriller smells sweeter than a carefully made hardback of the same year. The cheaper the paper, the richer the perfume of its decline.