There are several recommended textbooks for studying UPSC Philosophy Optional. These books cover a wide range of topics and provide in-depth knowledge and understanding of the subject. Here are some of the most highly recommended textbooks:

1. Indian Philosophy by S. Radhakrishnan
- This book provides a comprehensive overview of Indian philosophy, covering different schools of thought such as Advaita Vedanta, Nyaya, and Buddhism.
- It is a highly recommended textbook due to its detailed explanations and comprehensive coverage of the subject.

2. A Critical Survey of Indian Philosophy by C.D. Sharma
- This book is another popular choice among UPSC aspirants for studying Indian philosophy.
- It covers the history of Indian philosophy, including various schools of thought, their key ideas, and their contributions to the field.
- The book is known for its concise yet comprehensive approach, making it an ideal resource for UPSC aspirants.

3. Western Philosophy: An Anthology by John Cottingham
- This book provides an anthology of key texts from Western philosophers, covering major philosophical ideas from ancient Greek philosophy to contemporary thinkers.
- It includes works from renowned philosophers such as Plato, Aristotle, Descartes, Kant, and Nietzsche.
- The book is highly recommended for gaining a deeper understanding of Western philosophy, which is an important component of the UPSC Philosophy Optional syllabus.

4. An Introduction to Indian Philosophy by Satischandra Chatterjee and Dhirendramohan Datta
- This book offers a comprehensive introduction to Indian philosophy, covering various schools of thought, including Vedanta, Samkhya, Yoga, and Jainism.
- It provides a detailed analysis of the fundamental concepts and ideas of Indian philosophy, making it an essential resource for UPSC aspirants.

5. Philosophy of Religion by John H. Hick
- This book explores the philosophy of religion, discussing various theories and arguments related to the existence of God, religious experience, and the problem of evil.
- It provides a comprehensive analysis of different religious traditions and their philosophical underpinnings.
- The book is highly recommended for aspiring UPSC candidates to gain a deeper understanding of the philosophy of religion.

These textbooks provide a solid foundation for studying UPSC Philosophy Optional. It is important to read and understand these books thoroughly to grasp the concepts, theories, and philosophical ideas necessary to excel in the examination.