Nowadays, I bigan "extensive reading" its mean, 本を沢山読む. Its not difficult but easy. If you are not good at golf, you can't read the sentence written about golf. On the other hand, If you are good at golf, you can read it easily.
Extensive reading is not only increasing your reading skills but also being able to improve your speaking skills. That's why, when you read the book, you have to read aloud it. Then, your English skill will improve soon.
I recommend you the books which were published from OXFORD.
Try to read them as soon as possible!