I discovered this accidentally today. It may be documented somewhere, but I was not aware of it. You can take screenshots of your iPhone and store them in your iPhone's photo library, even if your phone is not hooked up to your Mac. I assume most people know that they can take screenshots using the Organizer in Xcode, but you can do it with just the iPhone and nothing else. I wish I would have known this when I was taking screenshots for the book.
Here's how you do it. Hold down the power button - the one that's on the top edge of the phone. While the power button is being held down, press and then quickly release the Home button (the round button with a rounded square on the front of the phone). You'll hear the click noise and see a flash, and when you go to the Photos application, you'll see your screenshot.