Hello Aspirant,

See if you have applied for February attempt of JEE-Main 2021 under General category then you will be considered under General category but yes if you want to change your category to Gen-EWS then you can do so in the application process of April attempt of JEE-Main 2021 but make sure you should have valid certificate for Gen-EWS. By the way the percentile which you have mentioned is more than good enough to get you any seat in any of the NITs either they are top known or average ones even on such percentile you will definitely qualify for JEE-Advance 2021 so I would recommend you to prepare your self for JEE-Advance 2021.

Some best books for JEE-Advance :-

@Physics

• Concepts of Physics by H C Verma (Vol-1 and Vol-2)
• Fundamentals of Physics by David Halliday/Resnick/Walker
• IIT JEE Physics by Arihant (35 years Chapterwise Solved Papers 2013-1979)

@Mathematics

• Objective Mathematics By R.D. Sharma
• Plane Trigonometry and Coordinate Geometry (Two Books) by S.L Loney
• Problems in Calculus Of One Variable by I.A.Maron
• Integral Calculus for IIT-JEE by Amit M Agarwal (Arihant Publications)
• A Text Book of Algebra For JEE Main & Advanced by Dr. SK Goyal (Arihant Publications)

@Chemistry

• Physical Chemistry by OP Tandon
• Modern Approach to Chemical Calculations by R.C. Mukherjee
• Organic Chemistry by O.P. Tandon
• Organic Chemistry by Paula Bruice Yurkanis
• Organic Chemistry by Morrison & Boyd
• Inorganic Chemistry by J.D Lee
• Inorganic Chemistry by O P Tandon

You can check out the link given below for best preparation tips :-

https://engineering.careers360.com/articles/jee-advanced-preparation-tips-physics-chemistry-and-mathematics

I hope this information helps you.

Good Luck!!