Three titles in the Captain Rawson series by Edward Marston have dropped to £1.90 at Kindle UK. From the blurbs, this looks to be almost as much a military fiction series as a mystery/thriller series, but
SYKM does list it, so I'm posting it here. (I am actually quite hopeful my son might like this, as it seems to be sort of similar to the Cornwell/Sharpe series and even a little bit to the Scarrow/Cato & Macro series in that there seems to be a little bit of mystery and a lot of battle in them...)
Anyway, titles on sale are:
Note: The first title in the series,
Soldier of Fortune, is available free to read via KU in the UK, if you have that, or it's £2.48 in the UK and $3.03 in the US, not too horrible prices, if you want to try the series in order...
Kindle UK:
https://www.amazon.co.uk/Soldier-For...dp/B006WB7IFQ/
Kindle US:
https://www.amazon.com/Soldier-Fortu...dp/B006WB7IFQ/