Not sure if I'm too late, but I have two recommendations!
C.F.W. Walther's "The Proper Distinction Between Law and Gospel." (It goes by a few other names)
I have spent the last two years (my Junior and Senior years of High School) in the paradigm shift from Baptist/Nondenom to Calvinist and finally to the Lutheran Confessions. This book was a key part in that. Walther's book is chiefly concerned with preaching the Bible correctly, using the Law and Gospel for their intended purposes. Law is to be proclaimed so that we all are deemed sinners, and Gospel is to be proclaimed as Christ's death in our place for the forgiveness of our sins and the redemption of our souls. Incapsulated in that is the doctrine of being Simul (tangent). Walther was amazing at making me consider how other theologies (esp. Reformed, Holiness and Catholic churches) confuse Law and Gospel in many ways.
A book to read concurrently with Walther would be "The Hammer of God" by Bo Giertz. I truly think this book is Walther's lectures in novel form. It tells the stories of three young pastors in Sweden during different time periods and their struggle against the varied challenges to the faith and ministry in their own times.
I hope this helps, I am not exactly an expert in Lutheran Theology, but I do think these books would do you good in your search for good theology and Lutheran distinctions.
Grace and Peace Brothers!