Walk into a used bookshop and you will encounter the unique aroma of aging books. The smell is loved by some, disliked by others, but where does it come from? A physical book is full of organic material that reacts with heat, light, moisture and - mostly importantly - the chemicals used in its production. The smell comes from the reaction of the organic material to these factors. Chemists at University College, London have investigated the old book odor and concluded that old books release hundreds of volatile organic compounds into the air from the paper. The lead scientist described the smell as "A combination of grassy notes with a tang of acids and a hint of vanilla over an underlying mustiness." Chemicals are found in the wood pulp that makes paper, and also in the ink for text and illustrations. From a chemist's point of view, the biggest reason for a book's decay is acidity -- and paper with too much acidity is common in books printed in the 19th and 20th centuries, hence their rapid deterioration and the smell. Books printed by the earliest printers have survived for 500 years because the purity of their paper. Telltale signs of this deterioration is browning - an all over discoloration of the pages - and foxing which are localized brown spots. Books can also react to external materials. A newspaper clipping stored inside a book can cause a reaction through its ink and cheap acidic paper. Books can absorb strong smells from the environment and this is commonly seen with tobacco smoke. The best way to store your books is in a cool, dry environment away from direct sunlight.
So you know, why old books smell. Also see this new research on why old paper turns yellow.