After the Patrick James' book proved so inconclusive (and complicated) I decided to check out his (and my current favored explanatory theory's) ideological forefather, Kenneth N. Waltz. His book, Theory of International Politics, seems to be to realism what Das Kapital is to Marxism, it provides the founding idea. I found it to be easier and to a large extent more applicable than International Relations & Scientific Progress and can generally only recommend it to students of international relations.
Waltz's argument is the more or less well-known and plain realist one. He believes in an international system that is state-centered. These states foremost care about their own survival and all their actions are fundamentally shaped by this belief. He furthermore puts forward the argument that a bipolar system is fundamentally more stable (as in peaceful, meaning less wars) than a multipolar world. Especially a tripolar world according to him is inherently instable, as two great powers could band up on the third one and divide the spoils (which does not bode well for the future I guess, since my arguments probably will be that we are moving towards a world just like that).
One last note, some of his observations while surely sound at the time seem anachronistic today, as he for example claims that interdependence in his day (1979) was lower than before the First World War. Today, with China financing American debt and the EU having become an ever more important trading partner this seems like a dubious assumption to say the least.
I will not expound further on this book, too many ideas, too much food for thought, and I am too tired, but read it (or read my master thesis once I am done, I will pick up on a lot of this), seriously, it is worth it.