Below are some of the references we often use in classes, or use ourselves. The internet varies in quality, you can trust this a curated list of high-quality sites (of course that is subjective, let us know what YOU think should be included.)

This is an interesting article, framed around the discussion of agile versus waterfall. However, it also touches on many other topics, like why the Agile Manifesto was needed, includes a great classification framework for Product vs Project Management, and discusses the role of a project manager in an agile organization. The article is long, but thought provoking. Enjoy!