First of all go through NCERTs because this will make your foundation strong, and then move towards the standard books as mentioned below;
For Prelims, you can refer the following books:
- NCERT Class XI and XII
- India's Struggle for Independence by Bipin Chandra
For Indian Culture, refer art and culture by Nitin Singhania
- NCERT Class VI to XII
- GC Leong's Physical Geography
- World Atlas
- NCERT Class XI
- Indian Economy by Ramesh Singh
International Relations & Polity ;
NCERT XII Political Science with current affairs
Indian Polity by Laxmikanth
- Verbal and Non-Verbal Reasoning by RS Agarwal
- M Tyra's Quicker Maths
Coming to Mains Exam, 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, it's always better to go through one book many a time, than reading many books one time, revision is the key here.
When it comes to interview, be thorough with your home state, your academics, hobbies, reading newspaper on a daily basis is mandatory for UPSC till the time of your interview as the panel will ask questions from latest happenings, if it's scheduled after budget session, then go through it and prepare well, if you have work experience then be thorough in that department as well, even if you don't know answer to any question, just say you're not aware and that's fine, move forth, and have faith in your ability, if you have made through prelims and mains, then you deserve to be in the interview, hence be confident.