Exploring Martin Luther's Theology and Work:

The Bondage of the Will
  Martin Luther
Martin Luther's Basic Theological Writings
  Timothy F. Lull (Editor)
The Theology of Martin Luther
  by Paul Althaus, Robert C. Schultz (Translator)
On Being a Theologian of the Cross: Reflections on Luther's Heidelberg Disputation...
  by Gerharde O. Forde
The Spirituality of the Cross: The Way of the First Evangelicals
  by Gene Edward Veith Jr.