Thanks to benjamin: "For the code box to come in you need to follow a link that activates it. For example go to http://www.finder.com.au/the-book-depository-promo-code and click their link and the box appears."
I stumbled across this coupon code after wanting to buy Maus (which is a great deal in itself for $19.07):
http://www.bookdepository.com/Complete-Maus-Art-Spiegelman/9…
For the real bargains, you can look here:
http://www.bookdepository.com/dealsAndOffers
Don't forget 1% cashback (existing customers)/5% cashback (new customers) via Cash Rewards!
I got some books from them two days ago :-(