Gatekeeper is the fourteenth and
St. Albans Fire is the sixteenth in Archer Mayor's really enjoyable (IMO) Joe Gunther police procedural series. Each has dropped to $0.99 as a US Kindle Countdown deal for about a day and a quarter more. Several years ago, Mayor ran through putting all of the early titles in his series (which he appears to have gotten the rights back to) on sale, but recently, there have only been scattered sales. But, anyway, it's nice to see a couple at this nice price.
Gatekeeper:
https://smile.amazon.com/dp/B00C3ESMVK
St. Albans Fire:
https://smile.amazon.com/gp/product/B00C3F0CSK/