The Book of Concord contains documents which Christians from the fourth to the 16th century A.D. explained what they believed and taught on the basis of the Holy Scriptures. It includes, first, the three creeds which originated in the ancient church, the Apostles’ Creed, the Nicene Creed, and the Athanasian Creed. It contains, secondly, the Reformation writings known as the Augsburg Confession, the Apology of the Augsburg Confession, the Smalcald Articles, the Treatise on the Power and Primacy of the Pope, Luther’s Small and Large Catechisms, and the Formula of Concord. You can read a copy of it online for free here. The Book of Concord is not Scripture, but  is a proper explanation of what Scripture has revealed to us about issues which concern the church.