Five hundred years ago a brash young monk single-handedly confronted the most powerful institutions of his day. His bold stand sparked the Protestant Reformation and marked one of the great turning points in history.

Martin Luther, a spiritual and historical giant, is loved and hated to this day--and for good reason. The anniversary of the revolution he helped start has produced a spate of doorstop-sized biographies. Luckily, today there exists a more accessible format that does justice to such a...