Here’s how it works: the oily extracts of the smells are placed inside really tiny bubbles of plastic, millions of microscopic bubbles.
How do they do this? By emulsion, the mixing of oil and water. When they furiously mix the extracts with water, the oil is broken up into very tiny droplets, at which point a gelatin mixture is dropped into the oil and water.
The gelatin settles around the oil droplets and forms a casing. The water is rinsed away, leaving gelatin-enclosed bubbles of smells. These bubbles are then mixed with an adhesive, and the adhesive/bubble slurry is printed onto paper.
When the book makes its way into your hands and you scratch the surface, you burst some of the bubbles and release the odor. Because there are so many on the surface, it’s hard to actually scratch them all away, which is why you can pick up a twenty-year-old Scratch-n-Sniff book at a secondhand store and still use it.
The process of producing the bubble-coated fragrance is called microencapsulation, and it was first used for carbonless copying paper, but instead of fragrant extracts, ink chemicals were placed into the coated bubbles.
When you place pressure on the paper with a pen point, the bubbles burst. The ink in the bubbles reacts with the chemicals in the bottom sheet of paper, producing a dark mark.