How operating systems work and how to write one.
Getting started with bare-metal assembly
How to create a hello world program in assembly that runs directly on top of UEFI (no operating system!). By Jonas Hvid.
How Operating Systems Work: 10 Concepts you Should Know as a Developer
Helicopter view of various central operating system concepts.
Writing a Simple Operating System from Scratch
How-to book on implementing a basic operating system.