Read about Martin Luther. He was a fascinating man. A good novelist could not have dreamed up a more interesting character.
Here are few books on Martin Luther well worth reading:
The Reformation: How a Monk and a Mallet Changed the World by Stephen Nichols
Martin Luther: A Guided Tour of His Life and Thought by Stephen Nichols
Here I Stand by Roland Bainton
Luther: Man Between God and the Devil by Heiko Oberman
The Genius of Luther's Theology by Kolb & Arand
Cambridge Companion to Martin Luther Donald McKim ed.