Probably the best book about a real Unix system is
The Design and Implementation of the 4.4 BSD Operating System,
Marshall Kirk McKusick, Keith Bostic, and Michael J. Karels.
To learn about Linux kernel internals, read
Understanding the LINUX Kernel
Daniel Pierre Bovet
To learn about Solaris, read
Solaris Internals: Core Kernel Architecture
To learn about Windows-based operating systems, read
Inside Microsoft Windows 2000
David A. Solomon
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,
The Emperor's Old Clothes.