Required:
Gerhard Weinberg, A World at Arms, pp. 170-86, 186-205 (pdf).

Murray and Millett, pp. A War to Be Won, pp. 91-109.

Overy, Russia's War, pp. 34-72.

Suggested:
Gerhard Weinberg, The Foreign Policy of Hitler's Germany: Starting World War II, 1937-1939 (Chicago: University of Chicago Press, 1980; Atlantic Highlands, NJ: Humanities Press, 1994).