For general purpose, the more you read newspapers, periodicals and books in English, the more improvement in vocabulary you will witness. At the same time, I personally feel that it is not sufficient to learn only the word or the phrase. We have to earn the usage also.
So far as preparation for CAT is concerned, there is no specific book for improvement of vocabulary.. The more mock papers you will solve, the more confident you would be.
Beware! I question everything and everybody.