There's another
$5 off any $5+ book offer. The offer expires
4/2/2018, so you'd better hop to it (Easter - hop to it, get it?
). Once you've redeemed the offer, the reward expires
4/14/2018.
I saw a small banner in the Play Store app a little way down on either the Entertainment/Movies-Music-Books page or on the Books page.
They apparently no longer allow you to use these offers on gifts. I first noticed this with the Valentines Day $3 off offer. I used to send a discounted gift from my secondary Play account to my main one. Now when I try that the discount is not applied.