Book Recommendation: Mac OS X Tiger for Unix Geeks
Seriously, how can you not love a book with this title? This is an update to the title Panther for Unix Geeks. It's got a lot of the same stuff, but it does add some interesting stuff about spotlight, launchd, and directory services in Tiger. It has lots of practical documentation for command-line access for user management, and disk utility stuff.
There's a ton of stuff about sharing files and printers with Windows and Linux, and guides for installing free x86 emulators (such as the free qemu). There's also a guide for setting up the PowerPC emulator PearPC on a Linux machine!
There are also some very good, if basic, tutorials on setting up and building open source programs. If you are a Linux or BSD geek who is new to Mac OS X, then this book will make you very comfortable, very quickly.
Here is the amazon link for the book.