Ah, Philby's story is one of those instances where reality is so much better than any fictional depiction can hope to portray. ^_^ And it's absolutely true that the old-boy network played a major role in allowing Philby to continue in his position for as long as he did. I'd also recommend Miranda Carter's Anthony Blunt: His Lives as an interesting tangent off this book if you're still interested in the general subject.

One question: Does the book focus mostly on the British side of the story, or does it go into the effect that Philby had on others like the CIA's James Jesus Angleton? Philby's betrayal hit Angleton particularly hard, so much so that Angleton spent the rest of his career in increasingly paranoid mole hunts that did much more damage than good, especially since it fuelled domestic espionage activities and all sorts of other nasty projects.