Thomas Hennessey & Claire Thomas 
Spooks the Unofficial History of MI5 From M to Miss X 1909-39 [EPUB ebook] 

The real history of MI5. ‘PO Box 500, London W2’ – the nondescript address from behind which one of the world’s most famous secret services hid: MI5. Drawing on previously secret sources, this book lifts the lid on Britain’s Security Service in its battle against German and Soviet espionage. It tells the sensational stories of the officers and agents and the enemies they confronted, from MI5’s creation in 1909 under the direction of Vernon Kell, Britain’s first spymaster. Building on the service’s wartime success, Maxwell Knight (‘M’), MI5’s charismatic and eccentric agent runner, penetrated Soviet and Fascist spy networks during the 1920s and 1930s. His agent, ‘Miss X’, was instrumental in breaking the Percy Glading spy ring run by the Soviets, while the beautiful Joan Miller and ‘Miss Z’ helped bring to justice Tyler Kent, who was passing information to the Axis powers.