Bach, M. J. The Design of the Unix Operating System. Prentice Hall International, Inc., 1986. A classic book describing the SVR2 kernel.
Goodheart, B. and J. Cox. The Magic Garden Explained: The Internals of the Unix System V Release 4. Prentice Hall International, Inc., 1994. An excellent book on the SVR4 kernel.
Mauro, J. and R. McDougall. Solaris Internals: Core Kernel Architecture. Prentice Hall, 2000. A good introduction to the Solaris kernel.
McKusick, M. K., M. J. Karels, and K. Bostic. The Design and Implementation of the 4.4 BSD Operating System. Addison Wesley, 1986. Perhaps the most authoritative book on the 4.4 BSD kernel.
Schimmel, Curt. UNIX Systems for Modern Architectures: Symmetric Multiprocessing and Caching for Kernel Programmers. Addison-Wesley, 1994. An interesting book that deals mostly with the problem of cache implementation in multiprocessor systems.
Vahalia, U. Unix Internals: The New Frontiers. Prentice Hall, Inc., 1996. A valuable book that provides plenty of insight on modern Unix kernel design issues. It includes a rich bibliography.