One of the hard problems of bulk book-scanning is the distortion in the scanned images arising from the bowed center of the book as it lies open. Google's clever solution to this is to paint the book with infrared light, and then use two infrared cameras to generate a 3D model of the book, which can be used to correct the scans.
The Secret Of Google's Book Scanning Machine Revealed (via Memex 1.1)
Turns out, Google created some seriously nifty infrared camera technology that detects the three-dimensional shape and angle of book pages when the book is placed in the scanner. This information is transmitted to the OCR software, which adjusts for the distortions and allows the OCR software to read text more accurately. No more broken bindings, no more inefficient glass plates. Google has finally figured out a way to digitize books en masse. For all those who've pondered "How'd They Do That?" you finally have an answer.