I got the promo message on my app, too.
HOWEVER...
I've now spent nearly two hours talking to 3 different people in Google Support, trying to figure out why I can't buy a $7.99 book using this promo.
Has anyone used it on a book that does not appear on the list? There's nothing that says that the title needs to come from that section, only that it needs to be an "eligible title" over $5. Even though the purchase page says that my $5 credit will be applied, I cannot get the transaction to go through.
Google is completely clueless, as well.