This video answers the question; “Why do old books smell?”. There are a number of reasons but a key part is due to chemical reactions. It’s a bad idea, for example, to store a piece of newspaper in a book as the acidic paper from the newspaper will damage the book over time.
At about the 40 seconds AbeBooks’ presentor mentions that your books may be releasing hundreds of volatile organic compounds into the air. Sure. We’re not talking about old RPGs becoming dangerous… but it’s still a freaky thought.