English, like any language, can be divided into three parts:

GRAMMAR (set of rules)
UNDERSTANDING (strengthens by usage)
VOCABULARY (meaning of words)
Books to strengthen each portion are given below:

GRAMMAR: You can read High school grammar by Wren & Martin or Plinth to paramount by Neetu or both.
UNDERSTANDING: You must practice 2 Reading Comprehensions daily from Previous Year Questions Chapter-wise (PYQCW) by kiran parkashan. And develop reading habit. Read at least 3–4 articles daily from any newspaper.
VOCABULARY: Some portion of vocab like one word substitutions and Idioms and Phrases can be memorized from PYQCW. And portions like Antonyms and Synonyms (in my opinion) should not be memorized but developed over a long period by consistently writing difficult words and their meanings in a separate notebook. And you must revise that notebook daily. Word power made easy is also a must read (at least 1/3rd of starting) book to understand how root words are used to form different words.
Summing up, i told you 4 books:

High School grammar by Wren and Martin
Plinth to Paramount by Neetu
Previous Year questions of SSC by Kiran Parkashan
Word Power made easy by Norman Lewis