For an introduction to the theory, Nand2Tetris[0] starts you out with nothing more than an xor gate from which you construct every other logic gate, implement a CPU, then in part 2 you build an OS on top of that. Although that sounds intimidating, it's actually incredibly accessible. I went in knowing nothing about the topic, and was able to implement the CPU by the end of the course, which was a major confidence boost. Also I found the lessons learned from the course (basically, layer simple systems with clear interfaces together to get complex systems) extended well beyond just CPUs and OSes to software design in general.

That uses a very simple educational architecture designed for ease of learning, though. If you prefer a good introductory resource for real-world operating systems, I just finished Georgia Tech's Intro to OS graduate course[1] and found it very accessible as well, and now feel reasonably comfortable with the theory when reading about OS topics elsewhere.

[0] http://www.nand2tetris.org/

[1] https://www.udacity.com/course/introduction-to-operating-sys...