On first acquaintance, we might be forgiven for reading The Horseman's Word as a lyrical evocation of a lost England, a poet's careful, often exquisite elegy for the landscapes that formed him. Those landscapes (coastal Norfolk, the semi-rural hinterlands of Surrey) are evoked with tenderness and care; the people, especially Garfitt's grandparents, are vividly drawn, and the attention to fine detail (the sensual feast of opening up a beach hut at the start of the season, for example) is close to Proustian. Yet this book is no idyll: hardship is recognised and given its due, while the clannishness and feudal nature of village life, familiar to anyone who has ever dwelt in the countryside, is mercilessly rendered: "One of the squires of Sedgeford, Sir Holkham Ingleby, wrote an affectionate little book called The Charm of a Village: but the charm rather depended on where you stood in the social scale. [His aunt] Ruth remembers another member of the squire's family calling at the house and asking, 'Is this Garfitt's cottage?' [Uncle] Frank answered her with the courtesy she expected, 'Yes, Ma'am,' but Ruth thought, 'Why can't she say Mr Garfitt? We have to show respect. Why can't she do the same for my father?'"

The casual harm done by the rural class system is not confined to disrespect, however: when a local schoolmaster makes a false accusation against Garfitt's uncles, the magistrates find themselves "in a dilemma: they should have dismissed the case but that would have made the schoolmaster look foolish. In the feudal society of the village as it was then, his position had to be preserved. They compromised by finding the boys guilty but giving them an absolute discharge." As a local gamekeeper points out, "gentlemen don't give a damn about our class of people. I've been with them so long I know just what they are."

In Surrey, however, Garfitt's father has risen to the position of lawyer, dedicating his spare time and most of his energy to the horse-breaking stables and riding school he has set up near Hersham, so that the family, especially Garfitt's mother, are keenly aware of belonging to a higher social stratum  and this has a very particular impact on young Roger's love life. Torn between intense, and rather peculiar religious impulses and an Augustinian attachment to the flesh, the teenager is attracted first to a stable girl, and then to an au pair who works for the family, but his class-conscious parents intervene at every turn, dismissing one girl and doing all they can to keep the other out of reach.

Not surprisingly, the young man grows up emotionally conflicted. The central part of the memoir, covering the author's teens and early 20s (coinciding, more or less, with the period when, according to Larkin, "sex began") is much taken up with romantic and sexual matters, as well as his emergence as a young poet in the Oxford of Peter Levi and John Wain, bringing in distinguished visitors such as Ted Hughes and WH Auden (who asked the dandified young Garfitt, when they were introduced, "How much a year do you spend on your hair?").

The description of Hughes is both funny and poignant: "Any student of animal behaviour would have been able to read our stance, the submissive, forward lean with which we asked our questions, and the distance we kept, holding to our little round tables while Ted Hughes leaned against the bar. He had just read to the University Poetry Society, his broad shoulders hunched over the book, his introductions terse to the point of impatience: "'The next poem is called "Otter". It's about . . . an otter.' The poems seemed to build like floodwater and break over the room, so that it came as something of a surprise when Craig Raine ventured, 'You don't actually read them very well, do you?' Hughes must have been surprised too but he didn't show it: 'Well, you get fed up with them towards the end.'" It's an exchange that strangely echoes the visit of the squire's kinswoman earlier in the book, but Hughes remains a rich and dignified presence, whose humility and sense of equality leaves Garfitt with the sense that "there might be some sense to us after all, some relationship between the elemental music we had just heard and our rushes of words to the head."

The Horseman's Word is full of such precise and tender portraits, both of incidental characters  the visiting poet, the chance encounter on the road, the latest in a series of beautiful, intriguing girls  and the author's family and closest friends. Garfitt's eye for the telling details of character, and his economy in relating them, evidence great skill and fine judgment, but it is in the second half of the book, when he recounts his descent into madness, that the full range of his narrative gifts emerges.

Madness is difficult to write, particularly from the inside, and the kind of madness from which the young Garfitt suffers  a walking-wounded, more or less functioning insanity that allows the sufferer to wander haphazardly from one terrifying situation to the next  is both the most dangerous to the sufferer and the hardest to convey. Yet Garfitt relates it so vividly that the reader enters into the madman's mind and sees the world from his point of view: the hallucinations, the illusions, the paranoid calculations, all are set out in the clearest prose.

This section of the book, one of the finest first-hand accounts of madness I have read, is a superb achievement; just as Garfitt is careful to sidestep the merely pastoral in his evocation of the English countryside, he scrupulously avoids the temptations attendant on writing about madness, so that, when we come to the point where the patient is reduced to the most basic level of existence, we go with him to his padded cell, to be broken, and to be healed.

John Burnside's A Summer of Drowning is published by Jonathan Cape.