Google Play is sending out emails to select accounts that have yet to purchase any eBooks. The email subject is "Just for you: $5 credit towards your first book."
For reference only, click here to see this promotion . If you did not receive an email, even if you are able to load this promotional page, you likely will not be able to get the credit.
Terms and conditions:
Enjoy a $5 credit towards the price of one book among the selection below. Your $5 credit will be applied at the payment screen. Redeem and use your offer by September 30, 2016 on Google Play from compatible Internet connected devices using a Google account. Offer limited to original recipients of promotional email. See Content Promotion Terms and Google Play Terms .