If war is just the continuation of politics by other means, what is the role of a secret intelligence service, and how does it change in times of declared and undeclared war? That's the question being asked - and mostly answered - by this history of MI6 during WWI, WWII, and the Cold War against Communism, which includes the period 1919-41 as well as post-WWII.
We get here an in-depth look at the heads of the Service, and its role in peace and war: from Boche to Bolsheviks is the title of one chapter, and it might as well have titled the whole book, really, if Jeffrey was trying to write a catchy story rather than an authorised history. There are lots of interesting vignettes here, but little on what you might be looking for - the Cambridge 5, the man who never was, etc: often because this was done by organisations other than MI6. The SOE for example were a wartime sabotage force, not an intelligence service.
At times it's a little dry, but there are interesting thoughts on the need for political independence of a secret service which in turn relies on its being nonpartisan. None of this is dross, or mere noise, but if you are looking for a chronicle of wartime adventure or secret operations and assassinations revealed...alas, this is not the book for you. The cover does say, reads like the script of a Bond film, but really its mostly just the bits where Bond meets with M (itself a play on C) that this book represents.
This is a great work, but be aware what it is not before you buy it.