The Book of Concord is the confessions of Lutheran Christianity.  At the website linked below you will find the complete text of the Book of Concord in English.

The confessions function a lot like the Supreme Court does in the United States by interpreting difficult-to-understand passages in the Bible, which in this case is analgous to the U.S. Constitution.

Of particular interest if you are considering the Lutheran way of being Christian is the Small Catechism, which was designed for parents to teach and explain the basics of the Christian faith to their children.  It presents the rudiments of the Christian life in the plainest possible terms.  The Large Catechism was written to direct the ignorant pastors Martin Luther found in his country at a sightly higher level, so if you are interested in knowledge beyond the basics, you might find this book edifying.

The Book of Concord was compiled over the course of some 50 years from writings of many of the Lutheran reformers, so it is important to know how to approach it.  Our church would view the Augsburg Confession as the heart of our understanding of a Biblically-faithful Christian faith, but the other sections of the confessions as important expositions and explanations of topics at greater length.