Start with high-confidence protestant christianity books, then narrow by subgenre or adjacent topics.
The Protestant Reformation
Hans J. Hillerbrand
The Oxford History of the Reformation
Peter Marshall
Across the Spectrum
Gregory A. Boyd, Paul Rhodes Eddy
The Reformation of the Sixteenth Century
Roland Bainton, Jaroslav Pelikan
Christian Spirituality
Donald Alexander
Reading Scripture With the Reformers
Timothy George
A Plain Account of Christian Perfection
John Wesley
Die Bibel
Martin Luther
