What are the books are the best for preparing jee mains in 2020?

I will write about the books I used. Note that I mostly relied on books and self study and so, I literally had a library of books with me. You are definitely not expected to purchase them all.

Physics:

• Concepts of Physics Vol. 1 and Vol. 2 - arguably the best book for beginners
• Mechanics Part-1 & 2 by DC Pandey - the best book for rankers
• Understanding Physics Electricity and Magnetism by DC Pandey
• Understanding Physics for JEE Main & Advanced Optics & Modern Physics by DC Pandey
• Understanding Physics for JEE Main & Advanced Waves & Thermodynamics by DC Pandey
• You might rather want to buy the complete set of books - Understanding Physics Series (Set of 5 Books) by DC Pandey
• Problems In General Physics by IE Irodov - buy this book if you want some challenge
• Resnick Halliday - a great book for theory and non trivial problems
• New pattern Physics Book by DC Pandey - a great book for problems!
  

• RD Sharma class 11th and RD Sharma class 12th - people tend to underestimate the school level books. But these are the ones that will build your foundation.
  
• Coordinate Geometry for IIT-JEE by SK Goyal - indeed a great book for Coordinate geometry. Its subjective problems are excellent
  
• Differential Calculus for IIT-JEE by Amit Agarwal - the best book for differential calculus for any competitive exam
• Integral Calculus for IIT-JEE by Amit Agarwal - the best book for integral calculus for any competitive exam
  
• Algebra for JEE Main & Advanced by Arihant Publication - not the best book, goes a lot beyond syllabus. If you are specifically preparing for JEE, then make sure to stick to syllabus
  
• Vectors & 3D Geometry by Arihant Publications - again, a great book
• Trigonometry for JEE Main & Advanced by Arihant Publications - trigonometry is not directly asked in JEE these days. Board level books are usually sufficient
  
• IIT Mathematis by ML Khanna - a highly subjective book for JEE. This book might get you selected in JEE, but isn't sufficient for a good rank
  
• Problems Plus In Iit Mathematics by A Das Gupta - I never used this book, but it is recommended by many toppers
  
• COURSE in MATHEMATICS for IIT-JEE (TMH) - I would say that this is the best book for JEE when it comes to problems. The algebra section is the best in this book
  
• A Practice Book of Mathematics JEE Main & Advanced by Arihant Publication - a great book for problems
  

• New Pattern Textbook of Physical Chemistry for Competitions (JEE Main & Advanced ) by OP Tondon - the best book for physical chemistry for any competitive exam
• Modern Approach to Chemical Calculations by RC Mukherjee - the best book for physical chemistry for problems
  
• Numerical Chemistry 22/e by P Bahadur - recommended by many top rankers
  
• Organic Chemistry by LG Wade - this is the BEST possible book for organic chemistry. DON'T buy OP Tondon organic chemistry. Many aspirants buy random books and then the book says something in 1 chapter and something contradicting in the other chapter. If you trust me, buy this book and see the magic of organic chemistry.
  
• Organic Chemistry by Morrison Boyd - you must read this book once if you are appearing for JEE. This is the so called "HC Verma" of organic chemistry
  
• Concise Inorganic Chemistry by JD Lee - a great book for inorganic chemistry. This book will teach you how to understand the subject rather than cramming it
  
• Textbook of Inorganic Chemistry for Competiotions (JEE Main & Advanced ) b y OP Tondon - a great book for problems
  

• NCERT Exemplar Problems: Solutions Physics Class 12
• NCERT Exemplar Problems: Solutions Mathematics Class 12
  
• NCERT Exemplar Problems: Solutions Chemistry Class 12
  
• NCERT Exemplar Problems: Solutions Chemistry Class 11
  
• NCERT Exemplar Problems: Solutions Physics Class 11
  
• NCERT Exemplar Problems: Solutions Mathematics Class 11