I have been comprehensively bowled over by Spies. I have never seen the dilemmas, confusions, excitements, insights, and incomprehensions of childhood better, more truthfully, done; and its balance of comedy and anguish - indeed the blend of comedy and anguish - is handled with exceptional delicacy. The fun is real fun, but it isn't allowed to cheapen or lessen Stephen's anxieties, fears, sense of his own unworthiness. (As an old man, he may have lost two of those, but not the third, I think.) All that would be enough to make this an exceptionally fine and unusual novel.
But Frayn also presents an adult story, imperceptibly humming in the background almost at the start, then thrumming more and more audibly as he brings it to the fore. When finally it declares itself openly, fortissimo and on centre-stage, one realizes that it has (and how it has) been at the centre of the story from the outset, though always - even at the climax - we get it through the consciousness of the boy.
The presentation of the adult story is an astonishing technical feat. Frayn shows superlative skill in the way he paces it - not just the rate at which the story comes forward, but the steps it takes to get there: the thriller-like excitement as it is gradually revealed, the discipline with which the revelation comes entirely through the experience of the boy Stephen, with nothing leaking around the edges, the growing revelation (starting long before we know what the story really is) of its sadness. It is an astonishing achievement.
The central adult story is heart-breaking. One is also sad for others, including the boy Keith and his poor limited frightened frightening father.
Frayn is never sentimental. He allows Stephen to be better in some ways than he thinks he is, and to have some significant decencies. But he also allows him to fail pretty seriously, letting down each of the two adult protagonists. The failures are shown as growing organically out of the condition of being a child, but they are failures nonetheless.
The long list of Frayn's novels has contained nothing else remotely like this. He continues to extend his range, taking new risks, exploring new territory.