I have just entered class XI and want to prepare for IIT. Please suggest me some of the best books for Physics Chemistry and Mathematics.

You can refer below recommended books for your preparation

PHYSICS

1-Concepts of Physics Vol I and II by H.C. Verma
2-Fundamentals of Physics by Halliday, Resnick & Walker
3- Problems in General Physics by I.E. Irodov

MATHEMATICS

1- Maths XI & XII by R.D. Sharma
2- Plane Trigonometry Part 1 by S.L. Loney
3- Plane Co-ordinate Geometry by S.L. Loney
4- Higher Algebra by Hall and Knight
5- Problems in Calculus of One Variable by I.A. Maron

CHEMISTRY

1- Physical Chemistry by O.P. Tandon
2- Organic Chemistry by Paula Bruice Yurkanis
3- Organic Chemistry by O.P. Tandon
4- Inorganic Chemistry- Chemistry XI & XII by NCERT