Google is running a promotion right now in Google Play Books where they are giving away a $5 credit which can be applied to the purchase of any single book priced $5 and up.
The offer does come with some strings attached; there's a limit of one per customer, and it is not available everywhere.
But if you do qualify then hey, five bucks.
I used it to buy the first volume in Marvel Civil War (which was loosely adapted into last year's Captain America movie). I had been wondering how the two differed, but didn't want to buy the graphic novels or sign up for a paid subscription service to find out.
And now I will know.
What did you buy with the credit?