God's No and God's Yes - CFW Walther - this is a series of sermons given by CFW Walther on Law and Gospel. As I recall this is an abridged version of the full lectures of the Proper Distinction Between Law and Gospel.
Augsburg and Constantinople - George Mastratonis - If you want to dig into the history of the differences between the early Lutherans and the Eastern Orthodox church, this is definitely the defining book. It is a series of letters between the Tubingen theologians and the Ecumenical Patriarch Jeremiah II. It is still the defining document from the EO perspective with Lutheranism. Be warned that you get into the weeds very quickly.
Examination of the Council of Trent - Martin Chemnitz - Probably still the defining book on Trent from the Lutheran perspective.
The Reformation - Owen Chadwick - When I took a class on reformation history at Xavier University, taught by a Jesuit, this was the main textbook. It is a good over view of the Reformation and the Counter-Reformation as a whole, not just on religious side but the politics, nationalism, and the wars that originated from the Reformation. Trying to study the Reformation solely from the religious side leaves out a lot of important components. For example, it wasn't just indulgences that kicked things off with Luther, but the background to Tezel. Albrecht, the Archbishop of Mainz and Magdeburg was trying to clear his debts for his selection as the Elector of Mainz. To clear this debt, he employed Tezel to raise the 10,000 ducats, which is the equivalent to about $1.5M or about the cost of a campaign for the U.S. Congress.
