Book scanners can cost thousands of dollars, but with a little ingenuity you can build one at home relatively cheaply.
As you can see in the video, the camera on the iPhone 4 is triggered by a Bluetooth fob mounted on the PVC pipe frame.
Aside from the cost of the phone (and a license for ABBYY software), I'd estimate that the total cost for the listed hardware parts and app is somewhere under a hundred dollars.
That's not bad for a rig which can be disassembled into a handful of components.
You can find more info in the relevant forum thread.