Child Wonder, By Roy Jacobsen, trans. Don Bartlett with Don Shaw

You've got a sister," his mother tells Finn, "a half-sister." This news heralds the first of several profound changes to Finn's life in the watershed year of 1961. Inevitably it makes him more aware of the occluded history of his father, who deserted his family for another woman, and died in a crane accident: "I can neither remember him nor the divorce, nor the accident, but Mother remembers for us both."

This new sister, six years old, will be coming to live with them – though why, and why now? – in their cramped apartment in one of Oslo's poorer districts. So the money his mother earns from her part-time job in a shoe shop may well prove insufficient. Taking in a lodger could help, and before long a halfway-suitable candidate shows up, Kristian, jack-of-all-trades with an intriguingly large vocabulary: "He had clear, calm, colourless eyes that watched not only Mother but me with a certain curiosity." Kristian pays on the nail, and is as fond of facts and figures as Finn could wish. At last his isolation at home and reliance for company on the tough children of the estate are interestingly offset.

Linda will surely provide further diversion. Her very arrival is both a disappointment and a portent of the problems ahead. Finn and his mother go down, as already arranged, to meet her off the 1.26 bus. But when it appears, nobody gets off. Mother has to dive inside and forcibly fetch out a little girl "who turned out to be small and fat and quiet with her eyes boring into the tarmac". Her continuing unresponsiveness bewilders both mother and son, the one falling back on a desperate sweetness of manner, the other unable to disguise his vexation. And back in the flat it exasperates Kristian who wants Finn's mother to bestow her attentions elsewhere.

At the heart of Roy Jacobsen's intricately worked novel, as rich in detail and implication as it is classical in construction and stylistic restraint, lies misunderstanding. It shows that this may not always be involuntary. Often, as here, the inability to comprehend a situation, to gauge another's personality, to identify one's own devouring emotions, has behind it a secret if unacknowledged purpose: to ignore the disquieting obvious, to smooth away the painful, to preserve untarnished images of those one loves.

The author is not specific about Finn's age (he must be about nine), and this decision mirrors his novel's governing idea: that to categorise people is both mistaken experientially and culpable ethically. This is most vividly and prominently apparent in the matter of Linda's mental abilities. How to explain her regular blankness before the adult world? Can any educational terminology do justice to it? Yet Finn does, most movingly, arrive at the essential truth. But with other truths he has greater trouble – particularly those concerning Kristian, depicted with eerie accuracy.

Jacobsen tells us in his foreword that Child Wonder is about an Oslo of "rather rough experimentation. Before oil. Before anybody had any money at all." This book is, even more, about the perennial sad irregularities of the human heart.