Hi, I would suggest you to follow the book list which is mentioned on the official website of Rau’s IAS Study Circle as I have prepared from it and believe me it was very helpful in cracking my mains exams.
I am mentioning the Geography optional subject books from which I prepared for my mains optional
Geography of the World
NCERT books on Indian and world Geography
Physical Geography : Savindra Singh
Geomorphology : Savindra Singh
Oceanography : Sharma and Vatal (NCERT)
Economics and Social Geography: Rupa Publications
Environmental Geography: Savindra Singh
Survey of Indian Agriculture : The Hindu Publications.
Survey of Indian Economy: The Hindu Publications
India Year Book : Publications Division