There are many excellent Unix internals books. Look for:

John Lions, " Lions' Commentary on Unix 6th Edition with Source Code", Peer to Peer Communications, 1996 ISBN 1-57398-013-7. This title was long-suppressed by AT&T until SCO bought the rights to System V and therefore became the copyright owner of Sixth Edition as well, and allowed it to be published. More info is at http://www.peer-to-peer.com/catalog/opsrc/lions.html.

Maurice J. Bach, "The Design of the UNIX Operating System", Prentice Hall 1986. ISBN 0-13-201799-7. Based on SVR3.0, but still sets the standards for the books on SVR4.0 and BSD. Often used as a textbook.

Steve Pate, "Unix Internals: a Practical Approach" Covers SCO OpenServer Release 5 internals.

Berny Goodheart & James Cox, "The Magic Garden Explained: The internals of UNIX SystemV Release 4.0", Prentice Hall, 1994 ISBN 0-13-098138-9. Builds on the Bach book but contains information on vnodes, unified VM system, and other things new to SVR4.

Leffler, McKusick, Karels, Quarterman, "The Design and Implementation of the 4.3BSD UNIX Operating System", Addison Wesley, 1990, ISBN 0-201-06196-1. This is the famed "Devil Book", named after the cute little demon on the cover named "Chuck". It is to 4.3BSD what Bach is to SVR3.

McKusick, Bostic, Karels, Quarterman, "The Design and Implementation of the 4.4BSD Operating System", Addison Wesley, 1996, ISBN 0-201-54979-4. An updated version of the above. Reflects innovations in 4.4BSD.

Uresh Vahalia, "UNIX Internals: The New Frontiers", Prentice Hall, 1996, ISBN 0-13-101908-2. Covers many of the new strains of UNIX that are unique including SVR4.2, Solaris, Digital UNIX, 4.4BSD, and Mach. Topics include kernel multithreading, multiprocessor and realtime systems, journaling filesystem, and modern memory management.