The first nine titles in the Kate Burkholder by Linda Castillo are $2.99 each today as part of the Nook Daily Find, matched at Kobo and Kindle US. This means that only the most recent (2018) and upcoming (next month) titles are not on sale.
The links are to the series - you'll need to poke around a bit to find the sale titles, since in some cases they are mixed in with some short stories and the full-price titles.
Nook:
https://www.barnesandnoble.com/s/kat...questid=409818
Kindle US:
https://www.amazon.com/gp/product/B0...ies_rw_dp_labf
Kobo US:
https://www.kobo.com/us/en/search?query=Kate+Burkholder