Specifically, books? I have a book that looks fine–the pages are not discoloured or anything–but it was in a box in a damp environment for a time and now it smells strongly of must.
What is must? Invisible mould? The beginnings of oxidation? Decay? Embedded dust?
Mildew, a relative of mold.
I have to wonder if there really are a specific set of chemicals that characterize “musty”. I know what old books smell like, and I know what it’s like to be in the basement of a house that has been abandoned for 40 years. Those are two different smells, but I call them both “musty”.
I think that perhaps we learn several different smells and categorize them as “musty”, just as we categorize different food smells as “spoiled”.