From the Summary: "This primer is designed to be a reference work rather than a linear narrative. Chapter 1 describes Congressional Budget Office's conceptual approach to analyzing the military's force structure and costs, and the following three chapters discuss the particular organizational structures and roles of the Departments of the Army, the Navy, and the Air Force. All of a department's major types of combat unit have their own entries, which discuss the size, cost, function, advantages, disadvantages, and past use of that type of unit. The final chapter includes similar entries for some major defense-wide organizations, such as Special Operations Command and the military's health care system. Each chapter also focuses on some special topics that are important for understanding the military's force structure, such as the integration of different types of units or the military's ability to conduct certain kinds of operations."
Congressional Budget Office: https://www.cbo.gov