The books you read for prelims would also be helpful for you to write your mains.One thing that is important here is that you should have command on these books and not just reading and that too in hurry just for the sake of completing it.
Here most important part is
For GS1 – GEOGRAPHY-NCERTS(+1,+2,GCLeong) are sufficient.
History-Ancient(6thNCERT+TAMILNadu board +1book) ,Medieval(7th NCERT+TN board +1 book),Modern(12th NCERT + Bipin Chandra’s Struggle for Indepence or Spectrum by Rajiv Ahir),World History(NCERT 9th 10th +1 +2) Art& culture +1 Ncert +selected chapters from Nitin Singhania
International Relations-The Hindu
Economics(NCERT 9th +1 +2(Macroeconomics))+ you can refer two yellow book from Vajiram.
Environment (NCERTS)Other syllabus part could be cover from Newspaper
Science-Inventions that were in Newspaper.
GS4-Yellow book of Vajiram on case studies…newspapers’ case studies can also be given to justify your stand .
one good newspaper like The Hindu or Indian Express- Make notes in GS1 2 3 And 4 format like the way the syllabus is organised.