Mostly focused on Master's degree related reading:

Stonewall in the Valley by Robert Tanner
Maurice's Strategikon translated by George Dennis
Warfare, State and Society in the Byzantine World, 565-1204 by John Haldon
Storm of Steel by Mary Habeck
The Training of Officers by Martin Van Creveld (have read this many times)
Technology and War by Martin Van Creveld
The Russian Way of War, Operational Art 1904-1940 by Richard Harrison