There are four areas you can work on to improve your English, which are Reading, Writing, Listening, and Speaking. If you have just finished this book, then you have been practicing your Reading skills. Did you read the book quietly in your head, or did you read the words out loud? If you read the book out loud, then you have practiced your Speaking skills!
You can ask someone to help read this book to you and practice listening to the words. By doing this, you would be working on your Listening skills. You can improve your English by working on your reading, listening, and speaking skills more, because practice makes perfect!