To get your 20% discount on all books in the 'Summer Reading' category, copy and paste the above code into the appropriate box when prompted during the checkout process. This will be after you have completed your address details.
This works whether you are paying by PayPal or your credit/debit card. The code is requested towards the end of the transaction, just before you finally pay.
You don't need to sign up for a personal account for this, but if you do, you can keep track of your orders and be the first to know about special offers in the future!