Understanding the philosophy of history is essential for Paper II of the UPSC exam. Below are some recommended sources that can help you gain a comprehensive understanding of this subject:

1. Hegel's Philosophy of History:
- Hegel's "Lectures on the Philosophy of History" is a seminal work in this field. It delves into the concept of historical progress, the role of individuals and nations in history, and the idea of freedom. This source provides a deep insight into the philosophy of history.

2. Arnold J. Toynbee's "A Study of History":
- Toynbee's monumental work examines the rise and fall of civilizations throughout history. It explores the cyclical nature of historical development, the influence of religious and cultural factors, and the role of challenges and responses in shaping history. This source offers a comprehensive analysis of the philosophy of history.

3. Karl Marx's "Historical Materialism":
- Marx's theory of historical materialism is a crucial aspect of understanding the philosophy of history. His works, such as "The Communist Manifesto" and "Capital," provide insights into the class struggle, economic determinism, and the role of material conditions in shaping historical development.

4. Oswald Spengler's "The Decline of the West":
- Spengler's work presents a unique perspective on the philosophy of history by focusing on the cyclical nature of civilizations and their inevitable decline. It explores the idea of cultural morphology and the organic development of societies. This source offers an alternative viewpoint for understanding historical processes.

5. R.G. Collingwood's "The Idea of History":
- Collingwood's work emphasizes the importance of understanding historical events in their proper context. It introduces the concept of historical consciousness, the relationship between past and present, and the role of interpretation in historical understanding. This source provides valuable insights into the philosophy of history.

6. EduRev:
- EduRev is an excellent online platform that provides a wide range of study materials, lectures, and notes on various subjects, including the philosophy of history. It offers a comprehensive repository of resources that can help you in your preparation for Paper II of the UPSC exam.

By studying these sources, you will gain a solid understanding of the philosophy of history, which will greatly benefit your performance in Paper II of the UPSC exam.