Im not sure if this is on just some new accounts. I received mine earlier today... no code just an email. you can try the link to see if you qualify.
"Valid for a $5 credit against the current on store price of a book from eligible titles on Google Play. Your credit may only be applied to eligible titles with an on store price greater than $5 before tax is imposed. Redeem and use your offer by January 31, 2018 on Google Play from compatible Internet connected devices using a Google account. Offer limited to original recipients of promotional email. "