<publication> "Source Code and Commentary on Unix level 6", by John Lions.
The two parts of this book contained the entire source listing of the Unix Version 6 kernel, and a commentary on the source discussing the algorithms. These were circulated internally at the University of New South Wales beginning 1976-77, and were, for years after, the *only* detailed kernel documentation available to anyone outside Bell Labs. Because Western Electric wished to maintain trade secret status on the kernel, the Lions book was never formally published and was only supposed to be distributed to affiliates of source licensees (it is still possible to get a Bell Labs reprint of the book by sending a copy of a V6 source licence to the right person at Bellcore, but *real* insiders have the UNSW edition). In spite of this, it soon spread by samizdat to a good many of the early Unix hackers.
In 1996 it was reprinted as a "classic":
[John Lions, "Lions' Comentary on UNIX 6th Edition with Source Code", Computer Classics Revisited Series, Peer-to-Peer Communications, 1996, ISBN 1-57398-013-7].