If you're looking for some insight into the theology of Luther's 95 Theses, don't miss B.B. Warfield (1851-1921), The Ninety-Five Theses in Their Theological Significance (1917). Originally published in The Princeton Theological Review in honor of the 400th anniversary of the Reformation, this is a fascinating study of the document by Martin Luther that launched the Reformation on October 31, 1517.