UPSC offers a diverse and distinctive questions from various areas in the prelims and mains examination. However there are certain books are must be relied upon in any case as they are very important for conceptual understanding of the subjects. As UPSC is moving towards more basics and opinion based upon those things, it is necessary to follow these books.
For Modern Indian History
1. India's Struggle for Independence by Bipin Chandra
2. Modern Indian history by Bipin Chandra
3. Indian Since independence by Bipin Chandra
For Indian Culture
1. Indian Culture by Spectrum/ NCERT
For Polity
1. Indian Polity by Laxmikant
2. Indian Polity by Sriram IAS Academy
For Economics
1. Indian Economy by Ramesh Singh
2. Indian Economy by Sriram IAS Academy
For Geography
1. NCERT 11th and 12th
2. Certificate Physical and Human Geograhy by Goh Cheng Leong
There are various books on ecology and environment and are of much importance looking into the fact that Indian Forest Service exam is same as that of Civil Service. So, there is a need for special emphasis on that.
For Environment and Ecology
1. Shankar IAS notes- As Recommended by many
2. Majid Hussain
3. PD Sharma
For Science
1. NCERT/ S. Chand of Class X
P.S. : The list of in-comprehensive and is not exhaustive. It is based upon my experience with books. Readers can take guidance from other sources as well.