Although this book will get you started with the Unix underpinnings of Mac OS X, there are many online resources that can help you get a better understanding of Unix for Mac OS X:
Apple's Open Source Mailing Lists
This site leads to all the Apple-hosted Darwin mailing lists, and includes links to list archives.
The Darwin Project
Darwin is a complete Unix operating system for x86 and PowerPC processors. Mac OS X is based on the Darwin project. Spend some time at the project's web page to peek as deep under Mac OS X's hood as is possible.
The Open Darwin project was founded in 2002 by Apple Computer and the Internet Software Consortium, Inc. (ISC). It is an independent project with a CVS repository that is separate from Apple's Darwin project, but it aims for full binary compatibility with Mac OS X.
Fink is a collection of open source Unix software that has been ported to Mac OS X. It is based on the Debian package management system, and includes utilities to easily mix precompiled binaries and software built from source. Fink also includes complete GNOME and KDE desktop distributions.
DarwinPorts is a project of OpenDarwin that provides a unified porting system for Darwin, Mac OS X, FreeBSD, and Linux. At the time of this writing, it includes thousands of ports, including the GNOME desktop system.
Like Fink, GNU-Darwin brings many free Unix applications to Darwin and Mac OS X. GNU-Darwin uses the FreeBSD ports system, which automates source code and patch distribution, as well as compilation, installation, and resolution of dependencies.
Mac OS X Hints
Mac OS X Hints presents a collection of reader-contributed tips, along with commentary from people who have tried the tips. It includes an extensive array of Unix tips.
Before Mac OS X, Stepwise was the definitive destination for OpenStep and WebObjects programmers. Now Stepwise provides news, articles, and tutorials for Cocoa and WebObjects programmers.
VersionTracker keeps track of software releases for Mac OS X and other operating systems.
MacUpdate also tracks software releases for Mac OS X.
FreshMeat's Mac OS X Section
FreshMeat catalogs and tracks the project history of thousands of mostly open source applications.