Philosophy is the most popular optional subject in the UPSC. Thousands of candidates choose philosophy as their optional subject.

To succeed in philosophy, one needs to find the best books to study.  A list of popular books for philosophy optional is provided by Careerindia.

An Introduction to Indian Philosophy  by Satishchandra Chatterjee

A Critical History of Western Philosophy: Greek, Medieval and Modern by Yakub Masih

A Critical Survey of Indian Philosophy by Chandradhar Sharma

Contemporary Indian Philosophy by Basant Kumar Lal

An Introduction to Political Theory by O.P. Gauba

Philosophy of Religion by John H Hick