Hello,

Mains is a second stage in UPSC Exam which it's more of a subjective type, you are required to write answers for Four General Studies Paper such as

GS-I includes Indian Heritage and Culture,History and Geography of the World and Society,
GS-II includes Governance, Constitution, Polity, Social Justice and International Relations,
GS-III includes Technology, Economic Development, Bio-Diversity, Environment, Security and Disaster Management,
GS-IV includes Ethics, Integrity, and Aptitude
Two Optionals,
One Essay,
Two Language Paper

In total there are nine papers in Mains, and each paper is of 250 marks, total adds up to 1750 marks.

You can refer the following books for four General Studies Paper;


GS I - For History , Class XI NCERT book with India's struggle for Independence by Bipin Chandra,India after Independence by Bipan Chandra,History of Medieval India by Satish Chandra. For Geography refer World Geography and Geography of India by Majid Hussain, GC Leong's Physical and Human Geography with NCERT Class XI book.
For GS II - Polity and International Relations , refer Indian Polity by Lamikanth,India's Foreign Policy by Rajiv Sikri
For GS III - For Economics refer Indian Economy by Ramesh Singh, read ARC report along with Economic Survey and Fiscal Budget.
For GS IV - For Ethics refer a book named Lexicon by Niraj Kumar, also you can go through study material of IGNOU.

Keep your resources limited always, read one book many a times and revision should be done on regular basis.