Why do old books smell?

That "old book smell" brings back so many memories, but what creates that distinctive scent?

Old books. (Credit: William Hoiles from Basking Ridge, NJ, USA (Creative Commons Attribution 2.0 Generic license.)

Every time I catch a whiff of that special old books smell, I am transported through time and space to the cool welcoming basement of The Strand Bookstore in New York City, where I spent many hot humid summer afternoons, searching for some u…