Well, Harry Potter books are for children. I wonder if children's books are better suited to foreigners, because they're less likely to contain strange vocabulary and long sentences.

On the other hand, most of the things that happen in Harry Potter books don't exist. And how many Japanese-English dictionaries have basilisks in them?

But I agree that reading genuine English text is helpful. The textbook for the "French in Action" video course has lots of clips of French literature at the end of every chapter.