Graham Greene's novel set in the British intelligence community in the seventies presents a very different view from, for example, those of Ian Fleming. There are no car chases, no explosions, and a surprising lack of cunning supervillains. As with most of Greene's work, in fact, the point is more that no one is particularly super. Instead of dashing secret agents, this novel presents a world of overworked and largely bored ones who drink too much, feel constantly disconnected from life, and are apt to make mistakes.
Into this environment, he throws the suspicion of a leak, closely followed by attempts to track it down that reveal more about the prejudices of the people doing the tracking than about the underlying conspiracy. It's a scenario that most people would have turned into a straightforward whodoneit or thriller. Instead, Greene turns the thing into one of his precise studies of the ordinary, throwing the banal at the reader until they're forced to understand how easily perfectly normal principled people could slide into betrayal, murder, or deception when confronted by a world with little in the way of clear cut boundaries.
The best of it is the sense of fallibility that cuts through the book, from the constant small errors of the characters' daily lives up to the final undercutting of each character's attempts to do what they see as the right thing. Greene backs that up by creating an environment where the lack of trust means that the reader isn't sure even at the end if they have the whole picture of what is going on. I picked this up half-hoping for escapism, but it's hard to be disappointed with the tense, perfectly sketched human drama that showed up instead.