The Sultan’s Seal is the first in the Kamil Pasha series by Jenny White, set in the late Ottoman Empire. It has dropped to $1.99 at Kindle and Kobo US (and discountable at Kobo). I’ve read and quite liked all three books in this too-short series, and this one, the first, was a finalist for a Historical Dagger award in 2006. Sadly the other two titles in the series don’t go on sale much at all in the US, so if you like this one, your library will be your friend, although I did notice that the second in the series, The Abyssinian Proof, has been £0.99 at Kindle UK for a while, if you’re up for traveling.
The Sultan’s Seal, #1: Kindle US | Kobo US
The Abyssinian Proof, #2: Kindle UK