What evidence and authority make The Song of Solomon a work of spiritual inspiration and not an erotic poem?

Is it not true that the place of this book in the Jewish bible was not established until sometime in the early Christian era? If that is so, is it not presumptuous to deem it as being among the so-called "Writings" and so equate it with Ecclesiastes, Job, Proverbs and Psalms? Is it not an incoherent erotic poem? What is inspired about that?