Books
Practical File System Design with the Be File System by Dominic Giampaolo. See also File System Construction Kit.
Think OS and The Little Book of Semaphores by Green Tea Press
Online courses
Computer Systems: A Programmer's Perspective, 3/E (CS:APP3e) by CMU
Snaptoken's tutorials walk through writing medium-sized software projects from scratch, step by step. Includes a text editor, garbage collector, and programming language. List of similar tutorials.
Write-your-own OS
The Adventures of OS: Making a RISC-V Operating System using Rust
Baking Pi. OS development from scratch for RPi B+ and A+ (not updated for later versions).
