I have a sort of minor obsession with Philby, due to early exposure to John le Carré, I think. This was a fairly boringly-written book (I had to keep reminding myself to focus on the page) but quite hilarious in places. The intelligence services seem to have been a near-farcical shambles from their inception till they rebuilt themselves in the wake of the Philby scandal.
The focus that all the writers (and John le Carré, in the introduction) put on betrayal is interesting, though. I find it difficult to get too worked up about betrayal in the context of the secret services: the way they function relies on betrayal, so clearly betrayal is not the problem - the problem is betraying the wrong side. This is sort of prodded at when they point out that Philby was a double agent at the time when he was probably giving most material to the USSR, which made it harder to identify him as a traitor. As far as the British intelligence services were concerned, Philby was acting the double agent, passing innocuous material to the Soviets, and passing himself off as a traitor for the benefit of Soviet intelligence. What was actually happening was that Philby was passing bogus information to the British and genuine secrets to the USSR. So it's highly problematic to claim that the worst thing about Philby was the betrayal: his whole traning was in betrayal and deceit.
There's a bit in Tinker tailor soldier spy where the traitor says "I have always believed the secret services are the only real expression of a nation's character." It comes from the traitor but it's something that George Smiley, who unmasks him, also agrees with; and in fact John le Carre said something similar in a recent BBC interview with Mark Lawson. I'm bemused by this fetishisation of the secret world. I can appreciate the interest that writers have in it: secrecy and betrayal make interesting stories. But the idea that the secret services are in some way an indicator of the state of the nation seems very strange: the view of an insider, maybe?