They also had participants in a different experiment take a whiff of a historic book from 1928, and were initially surprised to find that many of the people described it as “chocolate-y.” That ended up making sense, in a way, they said, because from a “chemical point of view,” coffee and cocoa contain “identical” compounds to those in decaying paper. Other descriptions included, “coffee,” “old,” “rotten socks,” and “mothballs.”

— Researchers at the UCL Institute for Sustainable Heritage have developed an "Historic Book Odour Wheel". Read more at The Odor ‘Wheel’ Decoding the Smell of Old Books at Atlas Obscura.