Theater of Spies by S.M. Stirling
This is the second book in his new alterate history series were Theodore Roosevelt wins the 1912 election.
Black Chamber operative Luz O’Malley Arostequi and her partner Ciara Whelan are sent back to Germany to get information into a new German invention that can turn the war in a really bad way for anyone who opposes the Kaiser. Lot of good basic spy craft material in this book. Some really good action sequences, but they come late in the book, when things go really wrong from a spy’s point of view. Stirling includes a nice James Bond reference in which will be more obvious to those who have actually read the books. There are references to actual historical figures, including Ernst Rolm, and nasty Austrian Corporal.