If your are a new to prep, Start with NCERT textbooks of relevant subjects & classes (also available online). Then some standard books like Spectrum/Bipin Chandra( History), Laxmikanth (Polity), Eco Survey, India Year Book (Selective study), Shankar- Environement, CA( from Newspaper & coaching like Vision) etc. Also govt schemes.
Always do selective study, and better start with reading/understanding previous year papers to analyse kind of questions asked.
For Mains, focus should be on Answer writing practice, to increase your marks.
Contact for further query. All the Best.