Back in the late 1930s joining the British Secret Intelligence Service, better known as MI6, could be a surprisingly informal affair. All you had to do was to make it known to the right people that you were interested and within days, sometimes hours, you were in! If, that is, you were qualified. Being qualified meant having the right background: good solid upper-middle to upper class family, educated at one of the elite public schools like Eton and Winchester, sound Oxbridge credentials, and recommendations from people who knew "your people" and thus knew you were "one of us." It seems rather obvious to us today that none of these "qualifications" had anything to do with whether or not someone would make a good spy, but that's how the system operated throughout the first half of the twentieth century.Harold "Kim" Philby and Nicholas Elliott were quintessential beneficiaries of the system. Both were from fine old families that had served the British Empire for generations. Both were public school boys who had gone on to Cambridge. Both found themselves at loose ends after leaving university, and both found it easy to get a place within MI6. Philby was older than Elliott, and the younger man looked up to him. Most people liked Kim Philby, as a matter of fact: he was charming, debonair, and sociable, able to party hard and work hard without ever getting a hair out of place. During World War II he was indispensible to the Allied war effort, and after 1945 he rose quickly through the ranks, seeming to position himself as a possible future "C", or head of MI6.There was just one problem. Philby was a Soviet agent. He had been recruited while at Cambridge, along with his friends Guy Burgess, Donald Maclean, and Anthony Blunt, and had served the Soviets faithfully for years, turning over names of agents and details of plans for espionage against the Eastern bloc. He was responsible for the deaths and imprisonments of countless Western agents, and no one seemed to have suspected a thing until the early 1950s. That's when Burgess and Maclean made their sensational escape to Moscow to avoid being captured as spies, and Philby came under suspicion of having assisted them. Even then his famous charm helped him out of trouble, and he eventually regained employment at MI6. It was not until 1963 that he was finally cornered and confronted with unassailable evidence of his treachery by Elliott, who had quietly and honorably served the West at MI6 for years. Philby defected to Moscow and lived the rest of his life behind the Iron Curtain, while Elliott continued his long and distinguished career in Britain.Ben Macintyre seems incapable of writing a dull book, and A Spy Among Friends ranks among his very best. I enjoyed reading the twists and turns of Philby and Elliott's careers, and about the colorful characters with whom they interacted. Much of the plot reads like a James Bond thriller, which is to be expected since Ian Fleming was himself part of MI6.If you are intrigued by Philby and his Cambridge friends there's a wonderful BBC dramatization called Cambridge Spies which traces their careers up to about the time Burgess and Maclean defected. There's also Anthony Blunt: His Lives by Miranda Carter and Tom Mangold's Cold Warrior, which is a biography of James Jesus Angleton, another friend and dupe of Philby's who became head of counterintelligence at the CIA.