In regard to English treatments of Luther, this set is one the most helpful in my entire library. The author, Ewald Plass, went painstakingly through Luther's writings (in their original languages) and compiled a massive anthology of what Luther thought about particular topics. It isn't a systematic theology per se, but there is a sense in which it's a topical systematic treatment of Luther's immense literary output. According to the Forward of the set, the project began in 1948 and finished in 1956.
What I find unfortunate is that detailed information as to exactly who the author was is not easily located online. Concordia Publishing says of the author,
Edwald Plass, a long-time Lutheran college professor, devoted his life to introducing laypeople and clergy alike to Martin Luther, a man's whose writings he thoroughly studied and read in the original language. His great work What Luther Says remains in print to this day as the most outstanding and thorough collection of the wit and wisdom of Martin Luther. This volume offers a storehouse of information about Luther, about those with whom he worked and struggled during the tumultuous years of the Reformation. In his book, Plass presents Luther's attitudes on many aspects of life, while demonstrating Luther's one overriding passion that animated everything he did: the Gospel of Jesus Christ. Let Ewald Plass introduce you to Martin Luther.
Founding Pastor, Rev. Ewald M. Plass, imparted to the congregation a strong support for foreign missions which remains a part of Mount Calvary’s ethos. Rev. Plass served two years at the Lutheran Seminary in Brazil and his passion for overseas mission work took hold at Mount Calvary.