Someone writing a fan-book on Harry Potter is NOT copying your stories.

1. Numerous authors have had commentaries and geeky fan books written as spin offs. What makes you so special?
2. I've seen a few His Dark Materials (far superior to HP) fan/commentary books, and I haven't heard about Philip Pullman suing anyone.
3. Every author is inspired by stuff other authors write. Some genres even have common themes (parallel universes, dystopian futures, etc).

What are you really worried about, JK? Your literary creation, or your royalties?

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EDIT: These were my initial thoughts at the story; they are not my views, as those are open to change when I realise I am wrong. :D
Judge for yourself whether you think he was copying, I'm not sure but then I wouldn't know. http://www.hp-lexicon.org/index-2.html

And Marina Hyde's informative Graunaid article is here.