The OS X book for shell divers

If you're either a new OS X user with some Unix background, or someone who has been using OS X for a while, and wants to know what's under Aqua, I recommend Mac OS X Unleashed, by John Ray and William C. Ray.

A few things are outdated with the release of Jaguar (Apple has switched printing over to the Common Unix Print System or CUPS), but there's good coverage of how to treat your OS X box as a UNIX box.