The Wonderbly books discount code 'MULTI20' is definitely the best one out there right now. Upon purchasing 2 or more books, you can save 20% on the total price. The good thing is that you can choose any 2 or more titles to redeem the code.
Also, a flash sale is going on right now, offering a straight 25% off on all titles. You have to use the code 'SPLASH' at the checkout.