Hey.

To improve your English Vocabulary, you can start with developing a habit to read the newspaper daily. Focus in the editorial section mainly, it will be really helpful for you to find new words. Other than that you can also read books. Pick a few new words each day and try to use them in sentencing. 

If you want to refer to books which focus on building your vocabulary them you can go for-

• Word Power Made Easy by Norman Lewis 
• 1100 Words you need to know - Bromberg and Gordon

Best of luck!