Sorry - I posed the question insufficiently well. I meant something independent of any particular library. Similar to the nationality of a human being chosen at random from the planet, where the answer is 'probably Chinese'.
It's not quite 'What is the commonest subject material found in a book', but more 'What is the most likely subject matter of a library book'. Both questions may have the same answer, but I suspect not.
EDIT: (Except, actually, the question is 'does anybody know if the above question is answerable').