+1 to HillRat's answer (especially hacking around in Codepen). On top of his, I want to plug the Mozilla Web Docs [0]. They're absolutely wonderful to reference for all your CSS needs, especially if you learn well from reading the docs. On top of that, they have excellent tutorials as well. I would consider checking out their one on CSS [1], you might find it easier to get through than an O'Reilly book.
A little unrelated but as someone who also loves their C++ and low-level programming I really enjoyed Lin Clark's articles on Firefox's Stylo [2] and WebRender [3], I think they do a great job of intuitively explaining the back-end behind CSS rendering, if that's of any interest to you.
These are great. Thanks for the breakdown - the whole webdev space has so much jargon that I probably should take the time to learn a bit of everything through the Mozilla tutorials and then pick up that book once I want some more