Grabbed the free book. I got a similar offer the other day, but for YA books.
YA list here.
I received this offer for "buying"
Timebound for free.
-Note: This was from Kindle First, which allows Prime members to pick up one of four soon-to-be released titles for free.
Adding: I ended up picking up The Bloodletter's Daughter because it peaked my interest and I had a giftcard to use.

Thank you for posting!