Popular books that teachers use at my school are The Hobbit and The Hunger Games series.
I would suggest going to a library's english novel section. Pick up a novel that seems interesting to you and read through the first page. If you find 2-3 words you dont understand, the book will have enough new words for you to learn but you will still be able to understand it, which is perfect. Dont get the book if you dont understand over 5 words or if you understand every word on the first page because it will either be to difficult and you will be confused when reading the book or too easy and you wont learn anything new.