Three omnibuses of books in Catherine Aird's Calleshire series, featuring Inspector Sloan (and Constable Crosby) are on sale in the UK - somewhat unusually for Open Road Media titles, which don't go on sale in the UK nearly as often as in the US. Each volume is £4.79 right now, which works out to £1.60 per book for the three-book ones, and £1.20 for the one with four books in it.
The Calleshire Chronicles Volume Three, contains
Parting Breath,
Some Die Eloquent, and
Passing Strange
Kindle UK:
https://www.amazon.co.uk/dp/B07GWHRJNC
Kindle UK/Smile:
https://smile.amazon.co.uk/dp/B07GWHRJNC
The Calleshire Chronicles Volume Four, contains
Last Respects,
Harm's Way, and
A Dead Liberty
Kindle UK:
https://www.amazon.co.uk/dp/B07GWCKBDF
Kindle UK/Smile:
https://smile.amazon.co.uk/dp/B07GWCKBDF
The Calleshire Chronicles Volume Five, contains
The Body Politic,
A Going Concern,
After Effects, and Injury Time
Kindle UK:
https://www.amazon.co.uk/dp/B07GWBVS4J
Kindle UK/Smile:
https://smile.amazon.co.uk/dp/B07GWBVS4J
This is one of my top five-ish mystery series...