"Can a man who is warm understand a man who is freezing?" Patrick Cockburn quotes Solzhenitsyn's question in the preface to this account of his son Henry's schizophrenia. The ensuing shared narrative brings the reader closer to such an understanding than anything else I've read.

Patrick's voice provides the framework. Jan, Henry's mother, adds extracts from her diary. Both write with intelligence, lucidity and compassion. But the sections written by Henry himself in prose that has, as Patrick says, "a sort of radiant simplicity", make the book truly special.

Henry first broke down aged 20. He was rescued by fishermen from Newhaven estuary, sent to a mental hospital and put on suicide watch. Patrick, a foreign correspondent hard to contact in Kabul, had little inkling of impending trouble, though Jan had worries. Henry, an art student, dirty and disorganised, went barefoot, dismantled his phone, believed clocks controlled his life, and was arrested for climbing a railway viaduct. But Henry had always been original and rebellious. What happened next was unimaginably, terrifyingly different.

The icy swim was not a suicide attempt  almost the reverse. Henry experienced "a spiritual awakening". He saw a vision of a golden Buddha. He swam under orders: "I felt brambles, trees, and wild animals all urging me on. It was as if they were looking at me and I could feel what they thought." The "reality" of the visions was visceral. When a "giant worm" came down from the heavens, Henry thought he could cut himself on its scales.

Patrick had polio in childhood; he understood physical illness. He was dismayed, now, to find that in his ignorance of mental illness he was not alone. Treatment for schizophrenia "seemed to be at about the level of treatment of physical sickness a century earlier".

Full recovery happened, if at all, apparently spontaneously. Drugs controlled symptoms, but worked better for some patients than for others. And why would patients co-operate with drug therapy and endure side-effects when they didn't believe they were ill?

Henry was gentle , never aggressive, but when he "thought the leaves of ivy brushing against my skin were telling me to pull down my trousers", how could the public see him as unthreatening? Released from hospital, he returned to university, but quickly deteriorated. His bare feet got septic, his "attraction to detritus" became acute. A year after his first breakdown, the police apprehended him, naked, in a neighbour's garden. Jan made the crucial choice, giving consent to his legal constraint in a mental hospital.

Aged 10, Henry wrote a poem about a caged lion: "Alas, alas, I can't go back, they won't let me back to my habitat." Would he have been better off left to his own devices in his newfound "habitat" of wastelands and alleys and talking bushes? Dangerous, yes, but many people, especially young men, live dangerously. Patrick himself made a career reporting from the world's worst trouble spots. (He asked himself whether this contributed to Henry's instability.) But Henry was capable of sitting naked for two days in deep snow. Incarceration seemed the only way of protecting his life. Besides, the Cockburns still hoped hospital would provide a "cure". It's worth noting (for future reference, as so many of us experience mental illness at some point) that Marjorie Wallace of Sane warned against the assumption that private care was superior  "Whatever you do, stick to the NHS." The courteous and fair-minded Patrick feels that overall the NHS has done well by Henry. But while detained in "secure" units, Henry escaped 30 times. "Life in a locked ward is miserably boring, and it is instant bliss running away." Even more importantly, "the trees were calling me and I had to [escape]".

The teenage Henry smoked a great deal more cannabis than his parents realised. Evidence indicates that cannabis can trigger schizophrenia. "The worst thing about smoking weed when you are a kid is that you never really grow up," writes Henry. "Your life turns into a sort of haze." But within the "secure" unit, cannabis was easily available.

As for prescribed drugs, getting even one unwilling patient to take them is a full-time job. Clozaril helped, but Henry "was able to devote more time to not taking his medication than the doctors and nurses had available to get him to take it". Increasingly, Henry suffered episodes, "tormented by forces that usher me hither and thither. 'Don't go in there,' they say, 'come nearer.' I hear the seagulls call." Hope of alleviation led him to accept Clozaril. The resulting improvement got him out of the locked ward into a "halfway house".

Henry describes hospital life with a novelist's power. "'I can hear what you are saying,' said the psychiatrist as, with the stroke of a pen, he renewed my section over my protests."

But he's still not sure he's ill, and the trees still speak to him. His heroic parents recognise that they "will always have to cope with the consequences of his schizophrenia. But that . . . is what families are for." A happy ending being unavailable, "the best we can hope for is that the story continues". The Cockburns have done a tremendous service in making their story available in all its horror, tenderness and beauty.

Charlotte Moore's Hancox: A House and a Family is published by Viking.

There are two principal ways of understanding mental illness, from the outside or from the inside. There is no physical test  no brain scan, no genetic read-out, nothing in the blood  that will identify whether someone has schizophrenia, bipolar disorder or depression. These are diagnoses established through behaviour and experience, as reported by the patient, or by others who know the patient. The psychiatrist's job is to identify a cluster of symptoms that point to a generic, underlying illness in order to suggest options for treatment. However caring and therapeutically engaged, the physician must regard the illness principally from the outside.

For the patient, of course, the opposite is true. Though he may be psychiatrically literate and capable of theoretical investigation of his condition, he will principally understand the illness subjectively, from the inside, as a struggle to be in the world.

But what about the spouse, or the sibling, or the parent  the intimate other seeking objective treatment for the loved one, yet also required to understand the mental torment of the illness as an experience in which they are subjectively implicated? That familial relationship  a state of being at once outside and inside of madness  is the least classified or understood in the history of medical psychiatry.

The first thing to say about Patrick Cockburn, as he tells the story of his son's acute and terrible schizophrenic illness, is that he is a very good reporter. A veteran war correspondent (most particularly in Iraq), he knows how to describe human suffering and to weigh the conflicting and often equally unreliable claims of perpetrators and victims.

It is no exaggeration to say that the job of writing about his son Henry's descent into madness is the toughest assignment of Cockburn's career. He openly admits that, until Henry's first psychotic breakdown in 2002, he had scarcely given a thought to mental illness, an area in which issues of interpretation and policy are as riddled with ideological baggage and theoretical complexity as any zone of political conflict. So he set about doing what a good reporter does. He conducted his research, he talked to the key people, he considered the various arguments and, in the resulting book, he attempts to synthesise these into a clear picture for readers whom he assumes will need informing just as he did.

The result is as good a primer into the current state of the psychiatric world as you will find. In plain, elegant prose, Cockburn walks the reader through what it means to be diagnosed with schizophrenia, the dilemmas of psychiatric classifications, the history of disorders and their treatments, and the current muddle of law and policy in mental health services. And he turns a nicely sceptical ear to medical scientists, politicians and mental health campaigners alike.

In one passage of delightful distillation, he exposes the paradox of those arguing to reduce "stigma" over mental illness, as if a bit more progressive empathy would somehow make insanity less frightening. "Fear of those believed to be insane was one reason the Victorians allocated so many resources to building mental asylums," he writes. It's a striking point, one he stumbled on while desperately searching for a hospital that could actually contain his son, who has an unerring gift for escape, and an irrepressible urge to run naked through frozen woods and rivers.

Yet this is only incidentally a book about psychiatry. Far more fundamentally, it is the story of a profoundly riven but deeply loving relationship between a father and a son. I have myself seen this happen at close quarters. Like Henry, my brother had a major psychotic break as a young man which led to years of living on the precipice between incarceration and extinction, and I also watched as my father, like Patrick, learned to accommodate and understand and finally even grow through the extraordinary, agonised delusional worlds inhabited by his son. I read this book, page by page, with a heart-thumping sense of recognition.

Because what makes it really special is the interspersing of Patrick's account with Henry's own. You hear the parent's version of watching his son embark on a journey into another world  talking to the trees, possessed of Biblical visions and torments, incarcerated and drugged  and then you hear the account from the ground.

And if there is a more lucid contemporary rendition of the experience of fully florid, schizophrenic psychosis than Henry's short, precise chapters in this book, I have not come across it. What makes Henry's writing so remarkable is that he remembers with precision both real events and the delusional ones that accompanied them. And, should you doubt a particular recollection, you need only cross-reference with his reporter-father's account of the same events.

Henry both believes in the reality of his hallucinations and, at the same time, recognises that they are his own confabulations. His schizophrenic universe is not merely a conjuring trick of gibberish. It interconnects with real events in a powerful and moving way, so that even the mad ditties and rhymes he invents have real-world sources. "Through and through and on to Peru/ Through every taboo and on to Peru," he chants. It could be one of Edith Sitwell's nonsense jingles, and yet it just so happens that Henry really had been to Peru for his cousin's wedding.

"I didn't think of it as an illness, but as an awakening," Henry writes. "I thought there was another side to the world I hadn't seen before." His mother, Jan  whose journal entries from periods of her son's illness are also included here  is the one who fully understands why Henry refuses medication. He resists the anti-psychotics, she says, "because he feels he is defending his whole identity and integrity and taking the drugs means that everything he thinks is wrong."

Yet, by the same token, neither parent is under any illusion about Henry's lack of remorse for the suffering he causes them, or even the hypocrisies of his illness. Happily admitting that he was routinely stoned as a teenager  cannabis being by far the most likely trigger for his illness  Henry blithely explains his resistance to anti-psychotic drugs: "I didn't agree with taking substances that would affect my mind."

This is truly an account of living with schizophrenia from the outside and the inside. It is a dispatch from the ultimate domestic war zone, a journal of suffering, a guide for others in similar distress, and a work of literary power. And the final paradox of Henry's Demons is that, even though there can never be a complete return from the farthest shores of madness, to lose a son (or brother) in this way entails not only regret and sorrow. By the end, one feels that Patrick has achieved stature and human dimensions through his struggle to reclaim Henry, and that Henry's illness has brought them both to a depth of filial understanding that  though they would never have chosen this path  they could not otherwise have known.