Hi,
The following are the best books for ias mains optional – geography which I have found from Rau’s IAS official website.
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
. Enviromental Geography : Savindra Singh
. Survey of Indian Agriculture : The Hindu Publications
. Survey of Indian Economy: The Hindu Publications
. India Year Book : Publications Division