Which is probably less relevant today in terms of directly understanding the implementation. But an interesting and enlightening read. Things were much simpler and fundamental back in the 1980s. It's easier to understand that way. Then layer on top.
And then there's xv6 , a small Unix running on vx32 from MIT for teaching purposes, full of comments, and available as a booklet that is directly inspired by Lions' commentary on the 6th edition of Unix.
I actually agree, though: to really appreciate the classics you should start with (Maurice) Bach. :-P