What is best book for IIT preparation for 6 class?

For class 6 you should refer to NCERT since all schools are supposed to follow that. 

Maths: RS Agarwal is good
Science if you are planning to prepare for JEE Pearson IIT Foundation Series
Hindi: Sachitra Hindi Vyakaran
EVS you can go for Oswaal

Best Book is NCERT to clear your basics and deep understanding of the subject.

For additional guidance, join EDUCARE.