the right time to start preparation is from second last year of graduation forCAT and joining coaching institute like TIME, CPLC, CATking. You need to be dedicated and passionate as to why you want to do MBA and make sure you devote atleast 2hours for preparation on regular basis. Results will only come from practice and interest in the subject. For Vocab stregthening, You need to select and cherrypick questions to get good score within less time. Hence, its advised to have accuracy and watch atleast one news channel everyday plus one newspaper everyday. You wont get accuracy by leaving questions, try to solve all problems initially and where you go wrong solve such questions more to make your weaknesses redundant, once you do that your confidence will boost miles away. Secondly, give maximum mock tests and make sure you dont repeat same mistakes again. once you're done with that you can then choose and select certain questions in final CAT exam to score great percentile within less amount of time for maximum accuracy. All the best for future endeavors.

It is good to start your preparations two years before you want to appear for CAT.

There are books like Word Power Made Easy by Norman Lewis, High School English Grammar and Composition by Wren and Martin etc to help you for your preparation in English and vocabulary.

These are some good business books and other novels that you can start reading

1. The 7 Habits of Highly Effective People Author: Stephen covey
2. Connect the Dots Author: Rashmi Bansal
3. Beyond the MBA Hype: A Guide to Understanding and Surviving B-Schools Author: Sameer Kamat
4. The Halo Effect: ... and the Eight Other Business Delusions That Deceive Managers Author: Phil Rosenzweig
5. Outliers Author: Malcolm Gladwell
There are various other books that you can refer to.

You can always read English newspapers and magazines to enhance your vocabulary.

All the best