I don't think it'd surprise anyone if I said I love the Harry Potter books. I started reading them when I was eleven and lived in Switzerland (so you can imagine my disappointment when I didn't receive my owl-delivered letter to Hogwarts. [England was right there!])
They were great. Plus, I've heard (so I'm not sure how true this is, though it makes sense to me) that one of Rowling's goals in creating the series was to start talking to a generation of kids, and then keep talking to them for the next several years. Obviously she didn't come out with a book every year (seven books over a decade), but that's pretty close. If you were eleven when the first books came out, you were around seventeen-ish when the last one did. Events that went on in Harry's life were roughly analogous to things going on in your own.
But the series ended. The supplementary books (Fantastic Beasts and Where to Find Them, Quidditch Through the Ages, and The Tales of Beadle the Bard) were all pretty short. Even with all the stuff on Rowling's website, Pottermore, and the world of the movies, once all that stuff came out, Harry Potter was over. We weren't going to ride the train to Hogwarts any more. Rowling is moving on as an author, and she has the right to do so. Still, haven't you always wanted to go back there, to Magical Britain? I know I have. That's why a book that I read this past weekend grabbed my attention so strongly. It's a piece of fanfiction called Harry Potter and the Methods of Rationality.
I loved this book. Not since the original HP series has a book grabbed my attention so firmly and refused to let go. I actually spent all of Friday night reading it (nine solid hours). It is in turns hilarious and heart-wrenching, filled with moments that make you want to stand up and cheer as well as ponder the new scientific concept this version of Harry has brought up.
Written by a researcher of Artificial Intelligence (a very smart fellow), HP&TMOR asks the question, "How would Harry's life have been different if his Aunt Petunia had A) married someone who loved Harry, and B) raised him to think as a scientist?" In this book, the author (Eliezer Yudkowsky) creates a world where Harry doesn't just accept the way wizardry works. He actively seeks to understand it through experimentation and gathering results. Why can't you Transfigure part of an object and not the whole? Do ghosts prove the existence of a soul? Why didn't Voldemort make a Horcrux out of an extrasolar space probe? And so forth.
The book is a wonderful read, and I enjoyed it as much as a young Colin did in his bedroom in Switzerland, staying up all night just to read one...more...chapter...
Here's the link to the page to read it online (or you could download the MOBI file to read it on your Kindle as I did: http://hpmor.com/)