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 these book and that too in hurry just for the sake of completing it.
Here most important part is
For GS1 – GEOGRAPHY-NCERTS (+1, +2, GC Leong) are sufficient.
History-Ancient (6th NCERT + Tamil Nadu board +1 book) ,Medieval (7th NCERT+ TN board +1 book), Modern (12th NCERT + Bipin Chandra’s Struggle for Independence 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))+ One standard book or class notes.
Environment (NCERTS) Other syllabus part could be cover from Newspaper
Science-Inventions that were in Newspaper.
GS4- Class notes plus newspapers’ case studies can also be given to justify your stand .
one good newspaper like The Hindu or Indian Express- Make notes in GS 1 2 3 And 4 format like the way the syllabus is organised.