Ask HN: What are your recommended reads that are available for free?

I recently stumbled across a link in another thread to "Economics in One Lesson" and thought it was incredibly interesting. https://mises.org/sites/default/files/Economics%20in%20One%20Lesson_2.pdf

What are some other interesting reads—whether PDF, website, doc, etc—that are freely available?

One of my favorites that I find thought-provoking is the "Procedural Content Generation in Games" book (http://pcgbook.com/).