Here I Stand: A Biography of Martin Luther by Roland H. Bainton

This biography of presents Martin Luther as complex man living in tumultuous times  in a clear, readable manner.
The book begins with Luther's days as a monk, his crisis of faith, and the development of his theology which led to his break from Rome, spearheading the Reformation. The central portion of the book deals with the conflict Rome and the Diet of Worms. The events of Luther's life, his words, and his work are always presented within their greater context. Much attention is given to the state of the Catholic Church during Luther's lifetime. The competing Protestant movements, humanism (particularly as pertaining to Erasmus), Anabaptism, the Peasant Revolt, and German nationalism are also put into context.
Bainton does a good job of explaining how Luther, Rome, and other parties differed in their views as to the nature of each and how they differed in their views as to how these points of the triangle properly related to one another - God to Man, Man to Church, and Church to God.

Well written and worth reading.