Apple has put together a Web page that describes some of the key Unix technologies expose by Mac OS X, including its Terminal (which gives you access to the Unix command line) and DTrace, an application debugging program written by Sun for its own version of Unix called Solaris.
If you want to learn how to leverage the Unix inside Mac OS X, though, the best place to turn is "Mac OS X for Unix Geeks," published by O'Reilly Media. The 4th version of the book, updated for Mac OS X 10.5 Leopard, just came out last week. According to the publisher,
You'll learn how to perform common Unix tasks in Mac OS X, such as using Directory Services instead of the standard Unix /etc/passwd and /etc/group, and you'll be able to compile code, link to libraries, and port Unix software using either Leopard and Tiger.
This book teaches you to:
ï¿¼ Navigate the Terminal and understand how it differs from an xterm ï¿¼ Use Open Directory (LDAP) and NetInfo as well as Directory Services ï¿¼ Compile your code with GCC 3 ï¿¼ Port Unix programs to Mac OS X with Fink ï¿¼ Use MacPorts to install free/open source software ï¿¼ Search through metadata with Spotlight's command-line utilities ï¿¼ Build the Darwin kernel
If that's all greek to you, then this probably isn't a book you should get. But if you just drooled all over the keyboard, then it's time to start hacking the kernel!