Its a tricky subject, no doubt, but very interesting. If one is interested, one must have idea of of international system, world affairs and world history i.e World War 2, cold war and other developments of international system. Advanced Publishers' 'International Relations; an introductory guide' is excellent work in this regard. But there are other books too, so be careful about the book mentioned above.
In addition to it, Microsoft Encarta Encyclopedia is very helpful also.