It seems you are going to go for CAT for the first time. Well then, you have to keep in mind the following points to fine tune your English part-
To improve your English the only way you can work on it is by reading more and more English. There is no other way.
What to read?
You can choose some novels based on your interest and other easy to understand books.
Buy ‘Word Power Made Easy by Normen Levis.’ This book can help you in building your basic vocabulary. It is seen that SSC exams vocabulary is at times hugely inspired by this book. The exercise helps you to never forget words once properly gone through it.
After completing this book you can also go for ’30 Days to More Powerful Vocabulary by the same author.
For English grammar, I suggest you High School English Grammar and Composition by P.C Waren. This book can be very helpful to you.
In your case, you can also go for Objective English for Competitive Examination.
How to Read Better and Faster. Read newspapers, especially the editorial section of The Hindu.
Book for Verbal ability and comprehension:
Verbal Ability and Reading Comprehension by Arun Sharma is a good book for verbal ability.
Full Study Material for CAT:
You can also download full study material for CAT exam in PDF from here.