It’s about Martin Luther teaching his barber, Master Peter, a simple way to pray.
You can read the whole book online here (“Preview the Book”).
Related: See Carl Trueman, “A Lesson from Peter the Barber,” Themelios 34 (2009): 3–5. Trueman’s article ends with this footnote (numbering added):
Martin Luther’s treatise on prayer can be found in the following works:
- Martin Luther, “To Peter Beskendorf,” in Luther: Letters of Spiritual Council (ed. and trans. Theodore G. Tappert; Philadelphia: Westminster, 1955), 124–30;
- idem, “A Simple Way to Pray,” in Luther’s Works (ed. Jaroslav Jan Pelikan, Hilton C. Oswald, and Helmut T. Lehmann; trans. Carl J. Schindler; Philadelphia: Fortress, 1968), 43:187–209;
- idem, “Luther the Confessional Theologian: A Practical Way to Pray (1535),” in Martin Luther’s Basic Theological Writings (ed. William R. Russell and Timothy F. Lull; 2nd ed.; Minneapolis: Fortress, 2005), 12–17.