There are several sources and books available that provide a collection of case studies for Sociology optional preparation. These case studies help students understand various sociological concepts, theories, and perspectives in a practical and applied manner. Here are some notable sources and books:

1. IGNOU Study Material: The Indira Gandhi National Open University (IGNOU) provides study materials for its Sociology optional course. These materials include case studies that cover a wide range of topics such as social stratification, gender inequality, caste system, rural development, etc.

2. NCERT Books: The National Council of Educational Research and Training (NCERT) publishes textbooks for Sociology at the higher secondary level. These books often include case studies that provide real-life examples of sociological issues and phenomena.

3. Sociology Optional Books: There are several books available in the market specifically designed for UPSC Sociology optional preparation. Some of these books compile case studies from various sources to provide comprehensive coverage of different topics. Some popular books include "Sociology: Themes and Perspectives" by Haralambos and Holborn, "Sociology: Principles of Sociology with an Introduction to Social Thoughts" by C.N. Shankar Rao, and "Modernization of Indian Tradition" by Yogendra Singh.

4. Research Journals and Articles: Academic research journals and articles often contain case studies conducted by sociologists. These case studies can offer valuable insights into specific sociological phenomena and help students analyze and understand them in depth. Some prominent journals in the field of sociology include the American Sociological Review, Social Forces, and Sociology of Education.

5. Online Platforms: There are various online platforms, such as EduRev, that provide a collection of case studies for Sociology optional preparation. These platforms offer curated content from various sources, including textbooks, research papers, and other relevant materials.

In conclusion, there are several sources and books available that provide a collection of case studies for Sociology optional preparation. These case studies help students gain a practical understanding of sociological concepts and theories by analyzing real-life situations and phenomena. It is recommended to explore a combination of sources to get a comprehensive coverage of different topics and perspectives.