- operating system in three easy pieces (https://pages.cs.wisc.edu/~remzi/OSTEP) is incredible for learning OS internals

- beej's networking guide is the best thing for network layer stuff https://beej.us/guide/

- explained from first principles great too https://explained-from-first-principles.com/

- pintos from Stanford https://web.stanford.edu/class/cs140/projects/pintos/pintos_...