Google is holding a sale in its ebookstore right now, but only for US customers.
If you head over to the Play Store home page right now you will see that one of the offers in the banner is for a 5$ credit on ebooks. The credit can be used once, and only on an ebook that cost more than $5.
According to the fine print:
Valid for USD 5 credit towards the current store price of a book from eligible titles on Google Play. Credit may only be applied to eligible titles with an on store price greater than USD 5 before tax is imposed. Offer ends January 2, 2018. Credit applied at checkout. Redeem by January 19, 2018. Limit 1 per user. Valid in United States only. Google Payments account required. Age restrictions apply. Cannot be used in conjunction with other offers or credits, except Google Play Gift Cards.
What are you going to buy?