Hello applicant,

If you are considering grammar ,so let me tell you Grammar is not something you can mug up.It needs practice and understanding the rules.But still for grammar there is no specified book you can go through any of the grammar book.

If you are asking for any of the books which can provide you fluency in English then read novels,old and new author books,for example ignited minds of APJ Abdul Kalaam etc.It depends on your taste what you like to read thriller,intellect challenging etc.

You can also download book reading apps for that.

Hope that helpsm