And my own email arrived just about an hour ago, several hours after the credit appeared in my Kindle account. My T&Cs are the same as @mitford13 listed above.
The potentially nice part is that I bought two titles this morning, one for $1.99 and one for $2.99, and it worked on them, even though they weren't more than $9.99 - although both were on sale, down from a "normal" price of more than $9.99. So I got them for free. And yes, I've checked the receipts for the two titles and they definitely show free.
(But this could also be potentially not so nice if this is really just three $5 uses up to $15, rather than truly multiple uses up to a total of $15. In that case, I under-utilized it a bit by using on $1.99 and $2.99 titles. But WTH, you can't always optimize everything, especially when they don't tell you the rules until several hours later
.)
As folks seem to have figured out, the titles it has appeared as if it would work on are all various imprints of Penguin or Random House.
Time to go do some more exploring...