I think the Harry Potter series by J.K Rowling might be a good idea, as the books get more and more complex as they progress. The first book for example is written far more simply (in terms of vocabulary) than the last one.

If you want something more advanced give China Mieville a try: he is a british sci-fi/horror author and I think he works with the English language as well as anyone is able to. Some really fascinating stories and wordplay.

Really advanced - Maybe give William Blake a go.