Whenever you are starting out your journey of UPSC preparation, must confine yourself to limited sources and revise them as often as possible. Moreover, always keep connected with the authentic sources. You must avoid using using multiple sources to prepare each subject. You need not to go beyond maximum two sources for any particular subject.
History
Ancient History 11th Std. Old NCERT Book Ancient India by R. S. Sharma Art and Culture 11th Std. Fine Art NCERT Book Indian Art and Culture by Nitin Singhania Medieval History 11th Std. Old NCERT Book Medieval India by Satish Chandra Modern History 12th Std. NCERT Book: Politics in India Since Independence A Brief History of Modern India by Rajiv Ahir
Geography
6th to 12th Standard NCERT Books Certificate Physical and Human Geography by G.C. Leong Oxford Student Atlas
Polity
11th Std. NCERT Book : Indian Constitution at Work 11th Std. NCERT Book : Political Theory Indian Polity by M. Laxmikanth
Governance
2nd Administrative Reform Commission (ARC) Report Ethics in Governance
International Relations
12th Standard NCERT Book : Contemporary World Politics Vision IAS Current Affairs Magazine The Hindu Newspaper or The Indian Express Newspaper
Economics
11th Standard NCERT Book : Indian Economic Development Indian Economy by Ramesh Singh or Mrunal Sir’s Lectures and Handouts Summary of Budget and Economic Survey
Environment
Environment Book by Shankar IAS Academy Vision IAS Current Affairs Magazine The Hindu Newspaper or The Indian Express Newspaper
Science and Technology
Vision IAS Current Affairs Magazine The Hindu Newspaper or The Indian Express Newspaper
Internal Security and Disaster Management
Internal Security Vision IAS Current Affairs Magazine Disaster Management 3rd Report of 2nd ARC : Crisis Management
Ethics and Integrity
Ethics, Integrity and Aptitude by Lexicon 4th Report of 2nd ARC : Ethics in Governance 12th Report of 2nd ARC : Citizen Centric Administration Case Study Material of Any Eminent Institute
Current Affairs
Vision IAS Magazines The Hindu Newspaper or The Indian Express Newspaper Summary of Yojana Magazines
PYQs and Others