These books tackle the design and implementation of Unix kernels. The first five discuss a specific flavor of Unix, and the later two focus on issues common to all Unix variants.
The Design of the Unix Operating System . Prentice Hall, 1986. A good discussion on the design of Unix System V Release 2. M. McKusick, K. Bostic, M. Karels, and J. Quarterman .
The Design and Implementation of the 4.4BSD Operating System . Addison-Wesley, 1996. A good discussion on the design of the 4.4BSD system by the system designers themselves. M. McKusick and G. Neville-Neil .
The Design and Implementation of the FreeBSD Operating System . Addison-Wesley, 2004. A good discussion on the design and implementation of FreeBSD 5. J. Mauro and R. McDougall .
Solaris Internals: Core Kernel Architecture . Prentice Hall, 2000. An interesting discussion on the core subsystems and algorithms in the Solaris kernel. C. Cooper and C. Moore .
HP-UX 11i Internals . Prentice Hall, 2004. A look at the internals of HP-UX and the PA-RISC architecture. Vahalia, Uresh .
Unix Internals: The New Frontiers . Prentice Hall, 1995. A superb book on very modern Unix features, such as thread management and kernel preemption. Schimmel, Curt .
UNIX Systems for Modern Architectures: Symmetric Multiprocessing and Caching for Kernel Programmers . Addison-Wesley, 1994. A superb book on the perils of supporting a modern Unix on a modern architecture. Highly recommended.