R.D Sharma ( 11th & 12th CBSE Board Book)[ Recommended for those who have very weak Fundamentals in Mathematics]
And Then Purchase R.D. Sharma ( Objective book for JEE Mains)
OR Error less Mathematics after that purchase Last 40 Years Problems of JEE Mains of Arihant Publication
For Reasoning, we would Recommend RS Agrawal as it is more than enough for aptitude Section and we recommend that you focus more on online tests of Reasoning section for your NIMCET Preparation. You can check out the NIMCET Topicwise Online Tests Series for more clarity.
For Computer Section I will Recommend PK Sinha as I am observing from last 3 years NIMCET ask atleast 7 questions from number System and Data Structure so focus only on these topics. Also check out our blog for the latest NIMCET Syllabus.
The horizons of English are to huge to compile. So our recommendation to you is not to follow a specific books. There would be something or the other that you can miss out. The best possible solution at this situation is to take an initiative to solve all Previous Years Papers of NIMCET 2008 to 2021. It would give you a complete idea about what to cover and what approach you can engulf.