Sweet Poison is the first book in the Lord Edward Corinth/Verity Browne series by David Roberts. It’s free right now in the US and UK. I read several (all?) of the books in this series back in the 2000s, when they were first published, and liked all that I read.
Sweet Poison was previously published by Constable and Robinson, but it appears that the author – or someone else – got the rights back, and now the first seven books in the series are available from a new publisher: dp books. And the remaining six are available at pretty decent prices: $2.99/£2.99 each in the US/UK respectively. So if you like this first free one, you won’t have to break the bank to read most of the rest of the series. (The last three don’t seem to be available in these new editions – so far, anyway.)
Oh yeah, and they are also KU if you have that…
Find Sweet Poison for free – as of now – at Kindle US | Kindle UK
Sweet Poison is now $0.99/£0.99 in the US/UK. Not free anymore, but still not too bad. Links in the OP are still good…