Lions book
From Computer History Wiki
The Lions book was the first work to document how the UNIX kernel works. It's based on UNIX Sixth Edition, and includes the complete source code for the kernel.
It was out-of-print for a long time, from shortly after it first appeared, after AT+T cracked down on it to protect their UNIX intellectual property. During this period, it became probably the most widely circulated item of samizdat in the West. It is now finally available again.
You can find more information on the book, and a PDF copy of it here.
