Martin Luther: A Man Who Changed the World by Paul L. Maier (A picture book about Luther.)
Luther & Katharina by Jody Hedlund (Historical fiction about the Luthers. This book was just released earlier this month. I can't wait to read it!)
Here I Stand: A Life of Martin Luther by Roland H. Bainton (The classic Luther biography)
Reformers in the Wings by David C. Steinmetz (Let's not forget about all those lesser known reformers!)
The Betrayal: A Novel on John Calvin by Douglas Bond (It's always fun to learn about an important historical figure's life through fiction.)
Getting the Reformation Wrong: Correcting Some Misunderstandings by James R. Payton (This sounds like a fun way to learn more about the Reformation.)