The three books of Ben H. Winters' Last Policeman trilogy, mysteries set in a world where an asteroid impact is about to destroy the Earth, are $1.99 (for book one) and $2.99 (for books two and three) today at multiple retailers. Here are
Amazon and
Google Play links. The books are also discounted at Barnes and Noble; no Kobo so far.
I've read the first two of these previously. They're pretty good, with a nice spin on the world-weary detective trope, and I'm looking forward to the third one.