This was a classic in theoretical perspectives about international politics in its day. Contributors to this edited volume included a variety of major figures, such as Hedley Bull.
The individual chapters deal with a variety of issues works characteristic of the era (published in 1966). In graduate school, this was an important work for us fledgling political scientists to read and understand.
The substance? Chapters covered a number of key issues of the time: Why the theory underlying international relations was so dicey; anarchy in international affairs; the balance of power; collective security; war as an instrument of policy.
The book does a nice job of exploring key issues of the time and raising important questions