Operating Systems: Three Easy Pieces by Remzi H. Arpaci-Dusseau & Andrea C. Arpaci-Dusseau (University of Wisconsin-Madison) is an excellent book and used by many universities worldwide. Extremely well written and it’s one of the only textbooks I’ve ever completed from start to end.
It’s also completely free: https://pages.cs.wisc.edu/~remzi/OSTEP/
I’m glad you appreciate it! It’s always fun digging into kernel internals and learning new things :D
I’m also open to criticism about the writing if you have any.