I understand this may be a tall order, since the authors most inclined to write on the subject would either be Protestant or Catholic and thus have an inherent biases. However, I am trying to find as objective a historical source as possible on Martin Luther and the Reformation. Any good recommendations?
First place to check is the reading suggestions list on the early modern period found here.
If you're looking for specific works on Martin Luther, there are a few good options:
Martin Brecht's three volume biography of Luther is the gold standard for works printed in English.
Brecht, Martin Luther: His Road to Reformation, 1483-1521 (1985).
Brecht, Martin Luther: Shaping and Defining the Reformation, 1521-1532 (1990).
Brecht, Martin Luther: The Preservation of the Church, 1532-1546 (1993).
Shorter biographies that are still quite good are: Kolb, Martin Luther: Confessor of the Faith; Marius, Martin Luther: The Christian Between God and Death; and Oberman, Luther: Man between God and the Devil.
For the German reformation, check the relevant chapters and works cited from MacCulloch's The Reformation: A History (2003) and Dixon, ed. The German Reformation: The Essential Readings.