A Shot in the Dark (Mydworth Mysteries, Band 1)

Sussex, England, 1929.
Mydworth is a sleepy English market town just 50 miles from London. But things are about to liven up there considerably, when young Sir Harry Mortimer returns home from his government posting in Cairo, with his unconventional American wife - Kat Reilly.
No sooner have the two arrived, when a jewel robbery occurs at Harry's aunt's home - Mydworth Manor - ending in one of the thieves being shot...and killed.
The local police are baffled by the case. But Harry and Kat have an edge in the hunt for the second thief: not only do they have certain useful "skills" they've both picked up in service of King, President and Country, they also have access to parts of English high society that your average bobby can't reach.
Because this Shot in the Dark...could have come from anywhere.