Google is currently running a promotion on the Google Play Books Store where you can apply a $4 credit to any ebook purchase over $4.
According to the terms and conditions, the offer is only valid in the United States and eligibility is based on purchase history, whatever that means.
I haven’t purchased any ebooks from Google in years so regular customers might not get the same offer; I often forget that Google sells ebooks since they don’t do much advertising and don’t offer any reading devices.
Here’s the fine print regarding the promotion:
Valid for $4 credit towards the current store price of an ebook from eligible titles on Google Play. Credit may only be applied to eligible titles with an on store price greater than $4 before tax is imposed. Open to participants based on purchase history. Credit applied at checkout. Valid in United States only, and for purchases made in US Dollars.
In addition to using Google’s ebook apps, Google’s ebooks can also be downloaded in ePub format, and sometimes PDF, so you can read them on any reading device that supports those formats, including Kobos and Pocketbooks. They also use Adobe DRM for protected titles.
Head over to the Google Play Books storefront to see if your account is eligible for the $4 credit. If so you should see a big banner at the top of the page.
I have no idea how long the promotion will last but you have 4 weeks to use the $4 credit after accepting the offer.