An Oblique Approach by David Drake and Eric Flint
The first in the Belisarious series. Alternate History by by two masters of the genre. Forces of vast power in the distant future locked battle, and decide to "fix the fight" by sending agents back in time to change history. The first wants to lock humanity into their vision of perfection. They send an AI back to days of the Eastern Roman Empire to create a powerful empire in Northern India, using their caste system to their advantage. Here is a hint for those who haven't read a lot of work by Drake, it involves introducing gunpowder long before it was used for weapons systems in our timeline. The second group sends a crystalline intelligence to contact a Roman General by the name of Belisarius. Conflict on a grand scale follows. A series of good adventure stories.
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.