I realize that I'm late to this party, but I just finished reading the novel-length fan fiction Harry Potter and the Methods of Rationality (HPMOR), by Eliezer Yudkowsky and I'd like to share a few thoughts on it. The story follows a re-imagined Harry Potter, in a world where he was raised by his aunt Petunia and the Oxford professor she married - no Dursleys in this one! As one might think, this sets the stage for a total revamp of the book universe.
Our protagonist has a deep affinity for hyper-rationalist thinking and uses it at every opportunity to get himself into and out of trouble, while the author uses it as a showcase for various rationality techniques. Bayesian thinking, cognitive bias recognition, and logical inference play central roles within Harry's toolkit, as does an experimentation-based empirical approach to wizardry that enables him to expose misconceptions that the magical community has internalized about the true nature of well-known techniques and entities such as Transfiguration, the Patronus charm, and Dementors.
Throughout the book Harry sticks to his Humanist Utilitarian ethics, emphasizing the moral duty of humans to reduce suffering of sentient beings regardless of their form. This is partially illustrated through internal dialogue, backed up by situations which cast a new light onto fixtures like the Sorting Hat. Ethical conundrums, such as whether the hat gains self-awareness only when it is on the head of someone who wonders about its self-awareness, add a interesting meta-level ethical concerns to the story in an reasonably unobtrusive manner; sometimes these are played straight, sometimes for laughs, as when Harry misses a meal due to his uncertainty about the level of sentience ascribed to magical plants and animals.
I finished the book quickly, within a few weeks of starting it, in part due to the immersive nature of the story and the short chapters. Things really started to pick up at around the 25% mark, as Yudkowsky began developing more intricate storylines around some interesting results in cognitive psychology. Longer arcs included "The Stanford Prison Experiment", "Taboo Tradeoffs", and "Roles", all built around common behavioral patterns observed and studied by psychologists in published studies. The focus on using Harry's cognition as a pedagogical tool really made the book more interesting to me rather than less. Despite the occasional heavy-handedness I felt that the discussion around rationalist techniques was a welcome addition to an entertaining story, rather than the other way around.
Overall, I would recommend HPMOR (and in the same vein, lesswrong.com to anyone interested in learning more about applied rationality and cognitive biases, who wouldn't mind seeing Ender Wiggin possess Harry Potter, or who has some time to kill between novels.