"The Design and Implementation of the 4.4 BSD Operating System"
"The Design and Implementation of the FreeBSD Operating System"
"Mac OS X Internals: A Systems Approach"
"Solaris Internals: Solaris 10 and OpenSolaris Kernel Architecture"
"HP-UX 11i Internals"
"IA-64 Linux Kernel: Design and Implementation"
Xenix manuals and later Steven's books were my introduction into UNIX world.
* http://jdebp.eu./FGA/operating-system-books.html
One of these days I shall get around to expressing my opinions, which as you can see are still missing. Indeed, the list itself is a decade out of date. (-:
I have some SCO UNIX manuals on the other side of the room as I type this.