If you ever wondered how Philby remained undetected for so long, ""the answer will be found in this novel."" You don't say. The Fourth Man is Primrose who for twice as long as Philby sustained his weekly communiques with the Russians, for no particularly good reason except general malcontentism and a wife who suggested the initial exchange. Finally a Russian defector, KGB passing for GRU, alerts the British to their man in the middle and they become more vigilant. Alan Williams has done this kind of thing with flair and an aura of actuality--Smith never rises above the anonymous apparatus of letters, numbers, codes, ciphers, and a couple who remain unnoticeable because they're so uninteresting.