There are two examinations through which you can take admission at ISI Kolkata for M.Tech CSE.
The written test conducted is of two types, i.e. Objective type questions will have mathematics paper (11th, 12th, and engineering level mathematics) and Subjective paper will have questions from Mathematics and one domain of choice.
One of my friends got admission to ISI Kolkata a few years back. Here are the reference books that she used for her preparation.
You don’t need to solve the book completely but make sure that you know which kind of questions are asked by referring to the previous papers and then solve them accordingly.
For the technical section, the questions are very basic and test the fundamental knowledge in the chosen domain. You will see the pattern and important topics from the previous year and prepare for it.
It is important to score as high as you can in the written test as it has a higher weightage.
In interviews, you will face a panel of 12-14 professors from every domain. They often ask questions based on your background and ask to prove the theorems from the technical domain. The interviews differ for each candidate. If your written test goes well, you will see your name on the final list.