Yesterday marked the fifth anniversary of the renaming of Android Market as Google Play, so Google threw an anniversary sale. Head on over to the Google Play home page and click the banner and Google will give you $5 off any ebook which costs more than $5.
The offer has to be accepted by 9 March and must be redeemed by 24 March. If you can't see the banner on Google Play
, here is the code: https://play.google.com/redeem?code=MRV4XTBWXNQW5S680T5BF0V
Android Police said that the offer was good until the end of the month, but that is not what I saw.
In any case, once you accept the offer, Google makes it easy to find matching books. A small message appears on valid book listings, telling you that you can redeem the discount.