There are three sections in the entrance exam and the level of maths is the same as what you studied in your 12th. The exam follows the pattern of most high school mathematics competitions (RMO, INMO, etc).
The three sections of the Entrance Exam consist of MCQ, A Subjective Test, and an Interview.
The books you must know are:
You should complete these all and do previous year’s papers of ISI/CMI RMO, INMO and IMO.
For Interview:
Do not overthink and do your preparation well. This will help you a lot. Be calm and try to think of cracking the exam and the latter part would be an easy task for you.