No single book covers the whole syllabus. During my CSS journey I consulted below mentioned books:
IR by Joshua Goldstein: An extremely useful and comprehensive book to grasp the subject of IR. However, it is too lengthy and may not be helpful near exams.
Recommended for General understanding of IR
IR by Kegley: Theories section from this book has no match. Concise and to the point. Recommended
IR by Prakash Chandra: Good for general understanding but the content is too old to be applicable in modern times. So not recommended
IR by Ikram Rabbani: Complete Copy paste and mixture of above mentioned books. Highly not recommended

Further take regular help from internet for making your own purpose built notes.