HTTP API Design: HTTP Fundamentals

I've been reading everything I can find on HTTP API design the past few months. I'm particularly interested in learning about the finer details of great, human centered APIs. It's been surprisingly hard to find a lot of great resources on the topic. So, I am writing up some of the basics for my own reference but also for others who are interested in the topic as I progress in my research. This article goes over the basic semantics of HTTP. I know many here are much more experienced and have operated at a much larger scale than I have (I build APIs for smaller factories), so please point me in the direction of resources you believe would be useful as I continue to study!