+1 for xv6. The commentary book is really well written, and is a straightforward introduction to a lot of important ideas. https://pdos.csail.mit.edu/6.828/201

+1 for xv6. The commentary book is really well written, and is a straightforward introduction to a lot of important ideas. https://pdos.csail.mit.edu/6.828/2014/xv6/book-rev8.pdf https://pdos.csail.mit.edu/6.828/2014/xv6/book-rev8.pdf