Dear Harmeet,

NIT Delhi takes admission through JEE Main. 

In order to prepare for JEE Main NCERT books are best as it helps to make your concepts clear. 

Apart, from NCERT you can recommended below books

Physics

HC Verma

DC Pandey

Chemistry

Organic Chemistry- Morrison Boyd/ O.P Tandon

Modern Approach to Chemical Calculations- R.C. Mukherjee

Numerical Chemistry- P. Bahadur

Mathematics

R.D. Sharma

Trigonometry, Geometry Books- S.L. Loney

Higher Algebra- Hall and Knight

Make sure you solve previous year papers and complete mock test series as well.

If you want to join coaching institute you can refer below

-Allen

-Vibrant Academy

-Aakash Institute

-Resonance

-Bansal

Joining coaching institutes also depends on your location.

Good Luck!!