The Congressional Research Service developed “a series of short primers to provide Members of Congress an overview of key aspects of the Department of Defense and how Congress exercises authority over it.” The defense primers, several of which have been recently updated, can be found here.
Other noteworthy recent CRS publications include the following.
Overseas Contingency Operations Funding: Background and Status, updated September 6, 2019
Congress and the War in Yemen: Oversight and Legislation 2015-2019, updated September 6, 2019
Afghanistan: Issues for Congress and Legislation 2017-2019, updated September 3, 2019
DHS Border Barrier Funding, updated September 6, 2019
Nonstrategic Nuclear Weapons, updated September 6, 2019