Harry Potter and the Methods of Rationality
by Eliezer Yudkowsky
Wow, I can't believe I read all this. (Don't judge me too harshly...) It was tough to get into at the beginning, but around the time the Sorting Hat scene happened, I found myself really getting into it and blazed through the other 900 (or so?) pages in a few days. The Sorting Hat scene is one of a few utterly 100% brilliant chapters, among others that are invariably clever but variably compelling.
BTW I never would have read this without the Kindle. I maintain, best tool ever invented for reading fanfic. And do I understand that it's still going past the point where the PDF leaves off? Yikes.