Or just get people to upload photos from the backs of the books in their library. Then extract from those the titles. Match them up against books that are missing in the digital library and are being requested by users.
Then send a message to the owner and ask them to scan this particular book.
Give an incentive based on both the number of requests and the speed of uploading the scan.
Could even allow requesters to set their own rewards. “I’d be willing to pay X to get this book scanned before Tuesday”.