Hello Bille, Greetings!

Well, I would suggest you to go for Wren and Martin. Wren N Martin is one book which is going to rectify your grammatical errors if you read the rules carefully and follow it in your speaking.

Also, you should do Word Power Made Easy by Norman Lewis to get the command on your vocabulary.

Besides read all the NCERT English Textbooks for class 6-12.

Thank you, best of luck.