If you are going for a Master’s degree just for the sake of getting good pay, then don't, it'll be a waste of your 2 precious years. Let me give you an overview: 

• M.Tech will not fetch you a 6-digit salary in an instant unless you are placed in a top company like Google. 
• If you're interested in going for M.Tech in CS then plan ahead- see what is asked in the ISI exam. You will find that it's mostly the B.Sc level Math.

If you are in Kolkata, then definitely go to ISI. There's a bookstore behind the college, it has a Math book especially for the ISI entrance test. You may buy and prep from that. 

My brother appeared in the written test for MSc Cognitive Sciences in February 2020. So, I can give you some insight about the same. 

The written test analyses your comprehensive skills i.e. the ability to implement the things you read. The section will contain comprehension questions. It will also include some of the paragraphs or questions based on the Cognitive Science Journals. 

Examination Pattern:

The written test is the same for everyone. 

• It contains 4 questions of comprehensive type and 1 of data interpretation. 
• The total marks for the examination are 25 and the duration is 1 hour. 

Preparation Tips:

• For preparation, you can simply take some random comprehensive type question off the internet and solve it. 
• In his examination, my brother had passages from pathophysiology of neurodegenerative disorders, case-studies on behavioral experiments using VR/AR, etc. 
• The questions are designed in a way that everyone can interpret them.

Be patient and stay motivated. All the best for your exam. 

My brother got admission to NIT Trichy last year. Here are his tips to do well in JEE Main. 

• You have 2 years to prepare for JEE Mains. Many students get good ranks even with 2 months of preparation. So, you have enough time to prepare. Start as early as possible. 
• JEE is not difficult. It just needs persistence and dedication. 
• Join a good coaching center, even better a dummy school.
• After you are finished with your basic books, i.e. HC Varma, A Das Gupta, etc. solve as many single choice questions as possible. You can use Arihant, GRB, Balaji publication books for this. 

Start preparing early and keep practicing. This is the only way to ace JEE and secure admission at NIT Trichy. 

Well, for Organic Chemistry, you can go for the following two books.

• Clayden: It is the best book with deep concepts in Organic Chemistry. Following this book, you can easily crack CSIR-JRF. However, chapter-wise, a detailed-study is needed from the starting and solving of the questions.
• Bruice: It is the best book for getting an entry-level understanding of Organic Chemistry.

It will be better if you start with Bruice and then Clayden.

Inorganic Chemistry:

• Huheey
• J.D. LEE: It is for beginners, and the questions from major topics in IIT-JAM come directly from Huheey; thereby, this book is a must.

Physical Chemistry:

• You can go through Atkins, however, it will be difficult to understand this book at the initial stage. For Quantum Chemistry, Atkins is a must. 
• You can also look at the various volumes of K.L. Kapoor. It consists of various questions along with solved examples related to Physical Chemistry.

Donald Mcquarrie's book is also a good option, it is the best book for spectroscopy and quantum. This book deals with sufficient terms and provides clear knowledge.

Solve roadmap problems for Organic Chemistry. For this, you must have a good command of the reaction’s name. Thereby, you can study Peter Sykes; maybe it is not comprehensive enough, but it will assist you in gaining in-depth knowledge for the reactions of Organic Chemistry.

Solving previous year’s questions will also be helpful. It will help you get an idea concerning the important chemical reactions. Also, check the organizing institutes of IIT-JAM to understand the demand of each institute.