Introduction:
Choosing the right books for the International Law optional subject in the UPSC Law exam is crucial for effective preparation. Here are some highly recommended books that cover the syllabus comprehensively and provide in-depth knowledge of international law.

1. "International Law" by Malcolm N. Shaw:
This book is considered the Bible for international law and covers all aspects of the subject. It provides a detailed analysis of the principles, rules, and doctrines of international law. The book is well-structured and covers topics such as sources of international law, international organizations, state jurisdiction, and state responsibility. It also includes case studies and examples to illustrate the concepts.

2. "Public International Law" by V.K. Ahuja:
This book is specifically tailored for the UPSC exam and is widely recommended by experts. It covers various topics like sources of international law, state sovereignty, recognition of states, and diplomatic relations. The book is written in a concise and easy-to-understand manner, making it suitable for beginners.

3. "International Law and Human Rights" by Dr. S.K. Kapoor:
This book provides a comprehensive understanding of international law and its relation to human rights. It covers topics like the United Nations and its agencies, international humanitarian law, and the role of international organizations in protecting human rights. The book also includes recent developments and case studies related to human rights violations.

4. "International Law" by S.K. Kapoor:
This book is another excellent resource for the International Law optional subject. It covers topics like state recognition, state succession, and state responsibility. The book also provides an overview of international organizations and their functions. It is written in a lucid and concise manner, making it easier for aspirants to grasp the concepts.

Conclusion:
Choosing the right books for the International Law optional subject is crucial for effective preparation and scoring well in the UPSC Law exam. The recommended books mentioned above, such as "International Law" by Malcolm N. Shaw and "Public International Law" by V.K. Ahuja, cover the syllabus comprehensively and provide a deep understanding of international law. Additionally, "International Law and Human Rights" by Dr. S.K. Kapoor and "International Law" by S.K. Kapoor are also valuable resources that cover various aspects of the subject. These books will help aspirants develop a strong foundation in international law and excel in the UPSC Law exam.