The short stories that Adam Mars-Jones wrote about Aids, collected in Monopolies of Loss (1992), led to him being hailed as “the Wilfred Owen of this new long-drawn-out and deadly trench warfare”, but that analogy is inexact. Owen famously wrote “My subject is War, and the pity of War”, whereas for Mars-Jones, Aids was less a subject than a setting. Similarly, Pilcrow, although telling the story of a severely disabled boy, is not really “about” disability. The author's principal concern as a writer has always been language, something that is becoming increasingly and depressingly rare among novelists. Critics sometimes complain that books are “overwritten”, but a far more common fault is that they are underwritten. Writing - as opposed to putting down words on the page in more or less the right order so that they make some sort of narrative sense - seems to be regarded by many practitioners as an optional extra.

Words are vital to John Cromer, Pilcrow's protagonist and narrator, who as a child in the early 1950s (brilliantly re-created) contracts Still's disease, a form of rheumatoid arthritis. This is misdiagnosed as rheumatic fever, the treatment for which is prolonged bed rest, whereas the best way of combating Still's disease is to keep the limbs moving as much as possible. Consequently, John is left virtually immobile, unable to bend at the hip and with severely restricted movement in his neck, arms, hands, legs and feet. Almost by way of compensation, his mind is supremely agile, and he flexes it in a way he never can his crooked body. His fascination with language is not merely a way of keeping boredom at bay, but allows him to explore, make his way in and exercise some control over a world that his body finds difficult to negotiate.

After spending some time in a fairly grim residential children's hospital, John goes to a forward-looking boarding school where “ABs” (able-bodied boys) are educated alongside and help look after their physically impaired fellow pupils. To set a novel largely among disabled children and adolescents is a perilous enterprise, but Mars-Jones inhabits this world absolutely, to the point where one forgets that he himself is able-bodied and merely imagining the circumscribed lives of his characters. Having described people living with Aids and (in The Waters of Thirst) life-threatening kidney disease, Mars-Jones clearly understands those whose physical security is under constant threat. At the same time, his characters are subject to the same dreams and desires, and are capable of behaving quite as badly as anyone else.

Although often tender, Pilcrow is rigorously unsentimental, and there is an audacious and often extremely funny matter-of-factness about the way it describes the various incapacities of John and his fellows and how they manage them. John himself ruthlessly exploits his disability in order to get into close physical proximity with those boys and teachers he fancies. Mars-Jones's absolute sureness of touch is also apparent in the early parts of the book, which contain a good deal about bodily functions, childishly experienced and expressed. People may find the material featuring “botties” and “tailies” and the production of “tuppennies” and “siss” embarrassing, but it is properly embarrassing in the way that children are embarrassing rather than in the way that adults writing about children can be. Throughout the book there is a skilful use of double perspective, with John vibrantly present not only as the boy undergoing these experiences but also as the adult looking back on them. His narrative swoops effortlessly between playful elaboration and childlike simplicity.

A glimpse of John's subsequent life and an explanation of baffling references here to his Hinduism have already been provided in Everything Is Different in Your House, published in Granta in 2001; further instalments are apparently in progress. It may seem risky for a writer who has made his reputation with a series of impeccably svelte volumes to embark on a capacious Bildungsroman, but if the rest of John's story is as beautifully written and as truly exhilarating as Pilcrow, most readers will be cheering him on.

Pilcrow by Adam Mars-Jones
Faber £18.99 pp525

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