The green card is either called a Y-38 (pretty sure) or Y-98 (not so sure) temporary export form. You will need to article your export on the card (you can only list 10 items... if you send more, you need to list on a separate sheet of paper and they will code that sheet with a serial number). Unfortunately, I think a customs official needs to confirm what you write on the card. You can have one do this at the nearest international airport.
Otherwise, when sending to CGC fill out the customs declaration sheet Canada Post gives you to send in as much detail as possible. When CGC returns, if they include this as evidence Canada was the country of origin for the books you should have no problems. This has worked for me every time.