Hello Aspirant,
Foundation IIT - JEE For Class 9-10 no doubt, is considered useful for students aiming to crack IITs. While the high competitive nature of JEE makes it imperative to be equipped with the most effective learning tools from an early stage, as a 9th grader you must have a crystal clear concept of the topics.
My advice in this case will be to first assess yourself and understand the concepts well and create solid foundation of the topics taught as in 11th and 12th you will have the same topics only taught with more depth and detail. Once you create a good base of the subjects, only then move to IIT Foundation books.
Also, there are many students (with average test scores below 85% throughout) who have cleared IIT just by rigorous study in 11th and 12th alone only one thing in common was a clear base that they acquired in their 9th and 12th.
All that said, if after completing all your subjects with clear concepts you still feel you have time, do go for these books because practice makes you perfect. But at the same time do not over do and destroy your social life.
Hope this was helpful.
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