The authority of the bible is undeniably just as valid as any other method in which God speaks. God's word exists in various forms. They are as follows:
1) The word of God as the person of Jesus Christ2) The word of God as speech by God - (for example in the form of powerful decrees that cause events to happen or even things to come into being. "And God said let there be light" Genesis 1:33) God's words of personal address - (for example when God speaks to people directly like when he declared the 10 commandments to the people of Israel4) God's words as speech through human lips (for example, him speaking through the prophets)5) God's word in written form (THE BIBLE)
One question to ask yourself is this: Do you think you would pay more attention if God spoke to you from heaven or the through the voice of a living prophet than if if he spoke to you from the written words of scripture? We must read and listen to scripture as if it's God speaking to us in his voice directly from heaven.