This is a vast oversimplification and as it happens, is untrue. I'm not going to defend Luther; but, this is untrue. And there is more to be said on this than meets the eye. Judaism - our roots - was and is a religion of the book as you like to say - to the extent that everything that God handed them was written down. It was done so that the message could be checked against what they new to be authentic. If it didn't line up, it didn't come from God. In effect you're bucking against what has always been as though it were something new. You are doing so in attempt to excuse the vast problems your sect has caused itself and others through philosophy and reason which is not your perrogative. In effect, you are being dishonest. Christianity is not a religion of a book - it is a religion of a message that happens to be contained in a book just as in OT times. And just as in OT times, if your message deviates from those who recorded the original, it lets us know you're a liar. Plain and simple. And without that measure, we'd have no sure rule to tell a prophet from a demon as we grow. It is our nature to question and be in doubt of ourselves. Scripture grounds us and assures us. And it lets liars betray themselves with thier own tongues.
You do realize that Judaism has an entire body of "oral Torah"?