Loci Communi - Common Places
The Augsburg Confession, 1530
Apology of the Augsburg Confession
Commentary on Romans
I found an author who is studied even less than Luther - Philip Melanchthon. The rabid Objective Faithless Justification dolts hate Melanchthon even more than they hate Luther. The major authors of the German Reformation are Luther, Melanchthon and the genius who studied under both men - Martin Chemnitz.