The first story I've seen about successful use of Google Print, and it's a recreational geometry book. Eric Gjerde used Google Print to find Greg Frederickson's book Hinged Dissections: Swinging and Twisting. He writes: "I can honestly say I would never have found this book if it was not indexed in Google Print. "

ETA: Metafilter thread on Google Print. Most of which seems to be comments of the form "this looks neat, why would you want to stop them from doing that?"