There are a couple of key words in the statement about Atlas Shrugged: influential and novel.
God and Man at Yale and Free To Choose are no where near as high in sales as Atlas Shrugged (influence) and I don't believe either one would be considered 'novels'.
I would say that both Freedman and Buckley were far more influential than anything Anne Rynd ever wrote. Since the policies promoted in both books were actually put into action by Ronald Reagan.
I didn’t know sales was the basis of influence after all checkers sells more than chess.