Hello Aspirant,
I took some survey from my seniors and faculty members. so, i can suggest you the best books for a average student to score good marks in jee mains as well as jee advanced 2021.
physics:
H.c.verma for theroy part of physics and cengage for objective part of physics.

Mathematics:
R.d.sharma for theroy part of mathematics and cengage for objective part of mathematics.

Chemistry:
NCERT books are the best for the theroy part of chemistry. Himanshu pandey for orgaric chemistry objective part and o.p tandon for the remaining objective part of chemistry.

If you can read all these books perfectly then you can have the ability to crack the jee mains as well as jee advanced exams.I Hope it will helps you.

Thank you.