I think reading is important for learning, but I don't know how much it will help your writing. Reading and listening are passive activities, but speaking and writing are active. Which means that many people can read much more than they can write, and they can hear more than they can speak. So if I were you, I would practice writing essays and having native speakers give you advice.
For reading, I would practice reading essays (to learn the proper form of an essay) instead of novels. I know it's certainly not as interesting, but I think it would help you more.
As for reading in general, If you see a word 3 or 4 times and still can't guess what it means, that's when it's time to look at a dictionary. (English-English dictionary of course!)
This is my advice to you if you are reading to practice for a test.
For general language, find novels that are interesting to you! I wouldn't recommend 'classic' novels such as Shakespeare or Dickens since they often use words or phrases that people don't use now. I wish I could read more... but I only read about one book per month.
Everyone learns differently, so what works for me might not work for you. But that's my advice. Good luck with your essay and happy reading!