Hey there.

According to me, if you seriously want to prepare for IPM, don't go after the number of books you need to prepare, rather you need to pick the syllabus and look for books that best prepare you for that syllabus. Cause it's not the quantity of books that you study from (ofcourse a minimum is essential), rather the quality of practice that you do from them.

So first of all, 11th and 12th maths syllabus books are a must. They will prepare you thoroughly for the quant section. Along with that, R.S. Aggarwal is another book that most people refer to for the quant section. For the verbal ability, Word Power made easy is a favourite. It is a great book that covers wide range of verbal ability enhancers.

All in all, whatever you do, make sure you adhere to the syllabus very well and do not distract from it.

Hope this help :)

All the best!