Though there are many books written on the subject but one must go for the book which covers the full course and provides you with some extra information , a rational persons should know that no single book can give you all these things in one volume.so one should consult at least three books to get good marks in the subject.Aspirants who have opted for European history can score good marks in this subject because they know all the basis of the subject and how European diplomacy and field of International Relations emerged and developed. so aspirants who do not have background of political science or European history will have to toil hard to get good position in the subject under discussion. current affairs are also included in the syllabus of IR so one must not forget to consult contemporary issues in International politics and general issues related to Pakistan in international environment.In my raw opinion following are good books on the subject:

"International Relations" by josua Goldstein (this book will cover the theoretical framework of IR)
"world politics since 1945" by peter Calvocracey (this book is also good for MCQ's)
"International Relations" by V.D Mahajan (this book will cover the part of course from 1914-1945 and also foreign policies of world powers before and after 1945)

May Allah be with us All.

Good Luck comrades