Hello,

For prelims you can go through NCERT book of class VI to XII this will build your base strong along with GC Leong's Physical Geography and World Atlas is must, When it comes to the Mains you can refer NCERT XII book, GC Leong's Human and Physical Geography, and World Geography and Geography of India by Majid Hussain, also consider going through previous year paper which will be beneficial, you can find the same at the official website of UPSC.