There was a time when you couldn’t be a Unix hacker unless you had a copy of John Lions’ A Commentary On The Sixth Edition Unix Operating System. It was supposed to be restricted to holders of a Unix license but was nevertheless copied and passed around. Many Unix hackers had six or seven deep copies of copies.
The book itself contains the complete source of the V6 Unix kernel along with Lions’ commentary on the code. For years it was, as the Jargon File points out, to only detailed description of Unix internals available outside of Bell Labs. In 1996, it was openly republished and is still available from Amazon.
Sixth Edition Unix was still simple enough that you can easily read and understand the whole thing. For that reason, it’s an excellent introduction for anyone who wants to understand Unix. Source code for even modern Unix versions is, of course, available now so someone dedicated to learning how the kernel works could start with the Lions book and go on to later versions.
I just came across an excellent on-line version of the Lions book provided by Warsus. If you’ve ever wanted to take a look at the book, here’s your chance to do so. It’s a great resource but, of course, mostly of historical value now.