Harry Potter and the Methods of Rationality. It is a significantly darker, adult-targetted fan-fic that is really well written. (In my opinion, the writing is significantly better than the original books.) I like it significantly more than the original series (which was fine). In fact, I'd say its one of my favorite books I've ever read, and I read a lot. The first few chapters aren't really representative, so consider reading to chapter 8, 9, or 10 or so before deciding you don't like it. The first chapters aren't bad, but the whole thing picks up steam and gets better and better as you read more. Expect a good number of plot twists. I should also mention that there is a lot of humor that you won't get if you didn't read the original Harry Potter books. And at a price of FREE, it is also a fantastic deal: Harry Potter and the Methods of Rationality
A couple more things: - One of the biggest themes throughout the book is rationality/critical thinking. - The author wrote under the pen name LessWrong, which is also a website about rationality. (Lesswrong.com) I haven't checked out the website.