MI6: The History of the Secret Intelligence Service 1909-1949, By Keith Jeffery

Following last year's publication of the authorised history of M15 comes the history of its arguably sexier and undisputedly more secretive partner in espionage and counter-intelligence. Any book closely monitored by the institution it's chronicling has to be treated with a fair dose of caution. One has to ask why, for instance, Keith Jeffery was allowed access to MI6's archives, and not someone else. MI6 retained the right of editorial veto; only a handful of agents' names are revealed; and the book closes in 1949 because anything after this was deemed too sensitive for the public domain.

Authorised histories aren't necessarily unreliable, though; quite the opposite. What one loses in playful conjecture, one can gain in measured scholarship. Judging by the stripped-down dust jacket, and the near-apologia by Jeffery in the preface that vast amounts of documents have long been destroyed, this is exactly what one is expecting over the 752 pages.

"Went to the office, remained all day, nor was there anything to do there," Mansfield Cumming, the founding chief of the service, noted in his diary in 1909 after his first day. What evolved, in charmingly haphazard fashion, was an organisation that by the end of the First World War – in spite of repeated attempts to usurp and amalgamate it – had more than 1,000 "staff and agents".

Jeffery casts a spotlight around the globe, highlighting the daring as well as the mundane, as these agents assist in halting firstly the threat of Kaiser Wilhelm II, then the menace of the Bolsheviks and Nazis. In astonishing detail, from the train watchers inside the La Dame Blanche network on the western front, to the Enigma codebreakers, it is a magisterial account of the two wars in particular, viewed via the prism of secret intelligence.

Winningly, it also entertains. There are multifarious references to such Bond gadgetry as "invisible ink" and rings that hold suicide tablets. It is fascinating to note that Cumming himself, travelling to a rendezvous in Paris, "was slightly disguised (toupée & moustache) and had on a rather peculiar costume". In a 1939 memo highlighting Nazi threats, alongside the creepily modern fear of a "bacteriological attack" was the altogether quieter concern of whether milk left on the Prime Minister's doorstep was bottled. "Milk bottles can be tampered with," it warns.

Failures are not ignored. MI6's involvement in the infamous forged Zinoviev letter left a lot to be desired, and Jeffery leaves open the possibility that MI6 might have been responsible for leaking it. Sometimes, the book is a story half-told. Conspiracy theorists will be tickled but left unsated over time-honoured questions such as the level of closeness between the German Admiral Wilhelm Canaris and MI6 during the Second World War.

Departing from the popular, hyped-up view of the intelligence services as institutions that callously bump off any threat to crown or state, Jeffery's account of MI6 during this early period is of an organisation with no agenda other than that of intelligence. It was mostly disinterested in policy. To what extent the archives will reveal this to be the case in the ensuing decades, Jeffery will have to leave to a successor, if ever one is elected.