Several titles in Gerald Hammond's Keith Calder series have dropped to $0.99/£0.99 at Kindle US/UK respectively, as Kindle Countdown Deals good for more than 5 more days. Links are below for both the US and UK. The UK link includes one additional title,
Sauce for the Pigeon, that isn't on Countdown in the US (at least for now). The UK link also includes three first-in-series Hammond titles that are pretty much permanently at $0.99/£0.99 pricing, but aren't pulled by the US link for some reason.
Many of these books do occasionally go free, briefly, from time to time, but with so many of them, the chances of any one going free at a given time are limited. So up to you if you want to wait...
Kindle US:
http://www.amazon.com/gp/search/?sea...price-asc-rank
Kindle US/Smile:
http://smile.amazon.com/gp/search/?s...price-asc-rank
Kindle UK:
http://www.amazon.co.uk/gp/search/?s...price-asc-rank
Kindle UK/Smile:
http://smile.amazon.co.uk/gp/search/...price-asc-rank