This is a collection of Yudkowsky's blog posts on LessWrong, and it's making the rounds now because it forms the basis for most of Harry's rationalist insights in the extremely entertaining and recently finished Harry Potter and the Methods of Rationality. (The last chapter of HPMoR went up on Ultimate Pi Day - 3/14/15 - last week.) I picked it up on the advice of some friends, and while the editing is less than perfect, it is overall an impressively well-organized and cross-linked treatise on how to reason.

At the heart of the book is the assertion that rationality is not about justifying why we are right, but thinking how we might be wrong, and paying careful attention to any evidence that puzzles us or could convince us to change our minds. The resulting philosophy is a combination of the falsification principle and the scientific method, applied to your own character. Highly recommended.