Four titles in the Inspector Adam Dalgliesh series by PD James, and also one non-series title, have dropped to £1.99 at Kindle UK.
The Black Tower, Dalgliesh #5:
https://www.amazon.co.uk/gp/product/B002RI90DQ/
Original Sin, Dalgliesh #9:
https://www.amazon.co.uk/gp/product/B002RI90IG/
A Certain Justice, Dalgliesh #10:
https://www.amazon.co.uk/gp/product/B002RI9ZXG/
The Private Patient, Dalgliesh #14:
https://www.amazon.co.uk/gp/product/B002RI91IA/
The Children of Men, non-series (looks more like dystopian fiction than a mystery):
https://www.amazon.co.uk/gp/product/B002RI9YDC/