Song of Songs: The Holy of Holies

In his excellent theological survey of the Old Testament—The Faith of Israel—William Dumbrell says that early disputes about the canonicity of the Song of Songs were virtually settled by a pronouncement of Rabbi Akiba. That's powerful, you say.  So what was it?  It was this marvelous word: "The whole world is not worth the day on which the Song of Songs was given to Israel; all the writings are holy, and the Song of Songs is the holy of holies" (m Yadayim 3:5).