Probably the best book about a real Unix system is
The Design and Implementation of the 4.4 BSD Operating System,
by
Marshall Kirk McKusick, Keith Bostic, and Michael J. Karels.
To learn about Linux kernel internals, read
Understanding the LINUX Kernel
by
Daniel Pierre Bovet
and
Marco Cesati.
To learn about Solaris, read
Solaris Internals: Core Kernel Architecture
by
Jim Mauro
and
Richard McDougall.
To learn about Windows-based operating systems, read
Inside Microsoft Windows 2000
by
David A. Solomon
and
Mark Russinovich.
To learn more about Mac OS X, go to
this site on the Apple page.
To learn about other interesting things from another master,
C. A. R. Hoare,
read
The Emperor's Old Clothes.