On a Christian view, God Himself has ultimate authority, which he has invested entirely in His Son, Jesus Christ. The bible MEDIATES that authority. Thus the bible is the Christian's penultimate authority. It's not been made entirely clear how a book that is mostly narrative is supposed to have authority, however. For instance, the biblical book of Kings is supposed to authoritative for Christians, but what does it mean for those stories to mediate God's authority over me? It's really strange.
The best account I've heard to date is that the bible is like a five-act play in which the first four acts are presented to us, along with the beginning of the fifth act and some author's notes about how the play is going to end up. Our job as Christians is to creatively act out the play in such a way that it makes sense in light of what has gone before and coheres with the sketchy ending indicated by the author.
Luckily, the author's best friend, who knows the author and his intentions for the play perfectly, is always available to the actors. He sometimes shouts instructions during rehearsals. Sometimes he stays silent for a while as the actors work things out independently. Sometimes he helps the actors understand the script. Sometimes the actors approach him for help and inspiration.
So that's the general picture of how the bible -- the play -- and the Holy Spirit -- the playwright's best friend -- together mediate the authority of God for Christians. Remember this is only something of a parable or an analogy, not a literal account of what I think is going on.
