From one perspective, Mac OS X is heresy. It's an Apple operating system with a command line. It doesn't hide its innards from tinkerers and hackers. It's not a closed box with a sticker that says, "NO USER SERVICEABLE PARTS INSIDE," like all previous Mac operating systems.

In short, it's a shocking and flagrant violation of everything the Mac has ever stood for.

As it turns out, nobody much cares. Newbies plug along, clicking Dock icons and dragging things to the Trash, without ever suspecting that only a thin shell of shiny pixels separates them from the seething, thrashing Unix engine beneath.

And power users are on Cloud 9.

So here they come, out of the woodwork: a nation of once marginalized Unix geeks, embracing the Mac, hailing Apple as the world's largest manufacturer of Unix boxes. These people are the pure of heart, the superusers who cluster at computer conferences with their PowerBook G4s and shoot bits of code at each other over the wireless network. Apple may have lost the battle for the corporate desktop, but with Mac OS X, it's picked up a new constituency of its own.

Part of the pleasure of reading this book comes from the hacks themselves: controlling iTunes with Perl scripts, using a Bluetooth cellphone as a wireless modem for your laptop, downloading files from the command line, and other preposterous stunts.

But much of the pleasure, too, comes from the pure, geeky fun the authors seem to be having. These are not serious adult males at the peaks of their writing careers ? they're five-year-olds stomping in puddles, laughing their heads off. These are people who don't for a moment question the value of turning the Mac into an Internet radio station powered by iTunes. Hey ? it's cool, and that's the greatest value of all.

These guys will lead you to favorite shareware programs, shine light on clever Unix command-line hacks, and show you how to turn off the brushed-metal window look of iChat and iSync. (Why? Because you can!)

This book might occasionally be over the head of many Mac fans. (If you want more general, less technical, everyday operating tips, try Mac OS X Hints, Jaguar Edition.)

But some people get as much a kick out of putting a computer through its paces as they do from everyday issues like productivity. Part of the spirit of hacking is doing things that the product's developer didn't quite imagine, finding the new and creative uses that only are possible to those who are willing to leave the beaten path. For the hackers among us, it's all about the thrill of discovery. If you're one of them, put on your backpack; you're about to go on quite a ride.

?David Pogue, Creator of the Missing Manual series