Even if they use Linux instead of *BSD, this book is the best way to learn OS internals and... well... 90% of it applies to Linux.
Many years later, has anyone written a Linux equivalent?
I have a book on Linux which includes the v0.x? source code and a commentary. It's at my other office, so I can't grok the title nor author. It's styled like the classic John Lions books for Unix which of course was the inspiration for Linux.
I keep telling myself I'm going to read it, but never get around to it. At this point, the John Lions book is probably just as relevant and probably much more educational.