NCERT books provide the basic foundation to prepare for any exam in general and civil services examination in particular. Since they are the government books so you can quote their data/information anywhere and that will certainty be the correct answer.
You can read from Class-VI to Class-XII books but 11th and 12th standard books are must, don’t ever try to skip them. You can refer old or new which one you get. For history old ncert books are preferred.
Download links are given below:
History
Geography
Economics
Polity
Science
Ethics
