It's because of paper and ink in old books. Paper is made of wood and wood is made up of organic compound like lignin and cellulose. These compounds breaks down under the presence of heat ,light and moisture releasing VOCs.
VOC stands for volatile organic compound. These compounds easily vapourise and mix in the air. Thus forming smell of old books. There are different types of VOCs.A volatile organic compound called vanillin gives vanila like smell. Another compound benzaldehyde gives almond like smell. Ethyl benzene gives sweat like smell. Hence, combination of various organic compounds makes old books smell so good.
