Angels in the gloom - Anne Perry

It's March 1916, and Joseph Reavley is on sick leave and finding recovery slow and hard. His sister Hannah is caring for him at home, and it's a delight to them both when Shanley Corcoran, an old friend, comes to visit. Corcoran confides in Joseph that he's come very close to completing an invention that will paralyse the deadly German U-boats. Soon afterwards, however, the leading scientist on that project is found murdered, and it's clear that someone has been betraying secrets to the enemy. Joseph's brother Matthew, of the S.I.S., comes down to investigate, and together the two men embark on a search that will solve the crime and lead them to the spy.