Today is the fifth anniversary of the Google Play Store. That's right, it has been five years since we saw the last of the Android Market - unless you had a very old device that never received the redesigned store app. To celebrate, Google is taking $5 off any book on Google Play Books - as long as it costs more than $5 to begin with.
To use this discount, open the Play Store app, switch to the books tab, and tap the 'Celebrate Play's 5th Birthday' banner. Then hit 'Continue' on the above popup, and you're good to go. You don't have to buy any books right now - you can wait until March 31, 2017 to redeem it.
Google also makes it easy to find out if the offer works on a book. A small message appears on valid book listings, seen above, informing you that it you can redeem the discount.