MI5 at War 1909-1918: How MI5 Foiled the Spies of the Kaiser in the First World War, Chris Northcott.

MI5 at War 1909-1918: How MI5 Foiled the Spies of the Kaiser in the First World War, Chris Northcott. 
A sober look at the performance of MI5 between its formation in 1908 and the end of the First World War, focusing as much on the internal structure of MI5, and the laws that allowed it to operate as on its activities and individual cases. This helps explain how MI5 achieved its successes, and also what sort of threats they believed they faced.
[read full review]