I agree with MrZain to some extent. I would suggest that one should only use academy written books for getting basic understanding of the subject. But to prepare CSS from them solely will not be helpful.

Regarding Sir Ikram Rabbani's book, I would suggest rather suggest to use FPSC recommended book written by Chris Brown. I've forgotten the title, sorry about that. Since you have time, you should also read another book from the recommended list on theories of IR. Peu Gosh's book is also very easy to understand.

The thing with academy published book is that everyone has them and everyone will be then using the same reference book, that would make your answer the same as everyone else's. By reading from various sources, one's own views widen and analytical skills also improve. So to make your paper stand out, it is better to use diverse sources. Also in papers such as IR, it is best to use books written by the scholars of that subject because then you've the advantage to quote them in your papers. I also recommend to either Google or just skim scam through some of the IR scholars work such as Joseph Nye, Fareed Zakaria (the same guy who hosts GPS show on CNN), Joseph Stiglitz, authors mentioned in the recommended list, and also to read on activists such as Noam Chomsky, Arundhati Roy etc. Chomsky's work is more on US's role as a super-power, Roy's "The Greater Common Good" essay, which is freely available, talks about the role of World Bank particularly in India's case. It is a very interesting essay that I recommend everyone to read.

To score well in IR use these tips:
1. quote scholars
2. apply theory
3. use facts, figures and statistics where ever applicable in your answer

My own score was 54 in paper-I and 78 in paper-II.