The Security Service, also known as MI5 (Military Intelligence, Section 5), is the United Kingdom's domestic counter-intelligence and security agency and is part of its intelligence machinery alongside the Secret Intelligence Service (MI6), Government Communications Headquarters (GCHQ), and Defence Intelligence (DI).

A bombshell when it came out, followed closely by an even bigger bombshell -- the revelation of what went on at Bletchley Park. I was in earshot when they went off.

Well written and entertaining. The facts need no embellishment. ( )

Written shortly after the end of the Second World War but not published for almost 30 years after that, John Cecil Masterman's account of the "double cross" system is meticulous, balanced and well organized. Since it was written relatively soon after the war ended, Masterman's perspective is still very immediate and there isn't too much hindsight to colour the impressions he has. As one might expect of an Oxford don, he writes well, with elegantly long sentences and touches of wry humour (one example: one of the double agents decides to steal an MI5 motorbike in an attempt to escape his guards; however, the motorcycle, "being government property, is not maintained effectively and soon breaks down"). The book also includes a very handy table at the end listing all of the major double-cross agents, their length of service, primary areas of interest, and reasons for discontinuing their use.

Because of its author and the time it was written, this is a particularly fascinating book. I would recommend it to those with an interest in the Second World War and espionage, with one suggestion: it may be helpful to read a more modern account first (e.g. by Christopher Andrew or Ben Macintyre), just to have the background fresh in your mind as you read. Masterman does provide ample explanation, since this book was originally written as a government report for an audience that may not have necessarily known the ins and outs of the system, but having the extra background really helped me personally. ( )

Finally, a definitive explanation for why Hitler did not believe the real invasion would be launched at Normandy. His "spies" led him to believe the landings would be at Calais. That message was the result of a strategic deception by the British intelligence. The author reveals the process of bringing a captured spy into an effective intelligence service, developing total empathy, and then providing credible messages.