For a while, Archer Mayor seemed to be cycling through putting titles in his Joe Gunther police procedural series on sale pretty regularly. But in the last couple of years, not so much...
Now he's got a couple of titles in the series on US Kindle Countdown for $0.99 each for about 7 more hours. So move fast if you want them.
Occam's Razor, #10:
https://www.amazon.com/gp/product/B00C3E0D9E/
The Marble Mask, #11:
https://www.amazon.com/dp/B00C3E9F7K