They say the only way to speak English is by speaking English. There are few books we can recommend for you:

Word Power Made Easy (by Norman Lewis)
This is the go-to book to improve your vocabulary. It is not a book to be read but a book to work with daily to enrich your vocabulary.

High School Grammar and Composition( by Wren and Martin)
It is a very good book for people who want to improve their grammar skills. It helps to understand and conceptualise different aspects of grammar and it is a must have book for anyone with poor grasp on verbal ability or who wants to make up with the basics of english.

Read books from different genres
The ultimate key to improve your verbal ability is to read extensively and to read books from different genres. This ensures that when the time for reading comprehension comes you are well acquainted with different writing styles and topics. Reading extensively also improves your reading speeding over a period of time. There are no shortcuts here.