Here are pdf links to the four volume biography of Martin Luther from James MacKinnon.
Volume 2: The Breach with Rome (1517-21)
Volume 3: Progress of the Movement (1521-29)
Volume 4: Vindication of the Movement (1530-46)
I found these volumes via this link. This set was one of the original biographies I used when I started researching the Reformation. I'm grateful someone has made them available online. MacKinnon's work is valuable because he was fluent in understanding the Denifle / Grisar (Roman Catholic) distortion of Luther. These are good historical source volumes to have.