In recent years, Flann O’Brien has often been characterised as the third member of the sacred trinity of Irish modernism, the Holy Ghost to James Joyce’s God the Father and Samuel Beckett’s God the Son. If so, he shares with the Holy Ghost a certain vagueness as to his identity: the press release that accompanies this book refers to him as ‘O’Nolan, or O’Brien, or Myles nagCopaleen or whatever his name may be’. Of these three personas, the first was a civil servant, the second a novelist and the third a satirical columnist for the Irish Times. Although they inhabited the same body, their relations were not always cordial: Flann O’Brien was effectively held hostage by his journalistic rival for two decades between the efflorescence of early novels and his re-emergence with The Hard Life (1961).

Expertly edited by Maebh Long, these letters are concerned largely with the travails of the jobbing writer. O’Brien was fascinated by St Augustine, but there are few confessions here: vitriol excepted, emotional candour is not his strong suit. Writing was a battle for O’Brien, and almost from the outset the career arc we follow is downward. O’Brien’s beginnings were brilliant, with At Swim-Two-Birds appearing in 1939 and his Irish-language satire An Béal Bocht in 1941. Much of the best work on his column, Cruiskeen Lawn, was done in the early 1940s. Thereafter came long years of creative blockage before the semi-resurrection of the 1960s, by which time the jousting punster of the early years had mutated into something altogether more unpleasant and angry.

O’Brien’s journalistic start came in the letters page of the Irish Times, where he would pile onto any topical row under multiple pseudonyms, often on opposing sides of the debate. This habit continued after the birth of Cruiskeen Lawn, and is represented here in a correspondence between O’Brien and one ‘Lir O’Connor’, each outdoing the other with ludicrous reminiscences about the titans of yore (such as when Ibsen’s wig fell in a soup tureen, and how he disguised his baldness through liberal application of ‘synthetic dandruff’, or was it ‘camel’s scuff’). These are among the funniest things in the book and rank alongside anything found in The Best of Myles.

The gappy chronology of O’Brien’s career is matched by the fitful appearance of his letters: the later 1940s flash past almost without a word. Anthony Cronin called his biography of O’Brien No Laughing Matter, and as the havoc wreaked on O’Brien’s talent by his drinking becomes apparent, the laughs acquire a scarifying quality. The first casualty was his civil service job, a post that had sustained and exasperated him in equal parts. O’Brien displayed a morbid sensitivity to the dead language of officialdom, dedicating regular Cruiskeen Lawn columns to what he called the ‘Catechism of Cliché’. Nor were the objects of his ire limited to a sloppy prose style. As he might have said himself: At great risk to what did fellow civil servants cross what with our hero? Life and limb; swords. ‘I PROTEST TO YOU IN THE STRONGEST POSSIBLE TERMS’, he thunders in 1952 during the long-running endeavour to invalid him out of the civil service for dysfunctional alcoholism. The campaign succeeded, freeing O’Brien to go full-time on his campaign against humanity at large. Publishers get harried and harangued, revenue agents are ‘gobshites’ who should ‘go fuck themselves’, everyone who works for television is ‘ex officio a cunt’, and insults are hurled in James Joyce’s direction (‘I’ve had it in for that bugger for a long time’), apparently for the crime of having been so nice about At Swim-Two-Birds shortly before he died. O’Brien’s was an overwhelmingly male world: female reference points are few, and one correspondent is told to ‘be a good girl’. O’Brien’s wife, Evelyn, probably best described as long-suffering, is hardly mentioned.

Unfortunately for us, the last years of O’Brien’s life are also the best represented. It is an abiding mystery why O’Brien was so sanguine about his chances of reviving his career in the 1960s with two below-par novels, The Hard Life and The Dalkey Archive, while leaving his masterpiece, The Third Policeman, in the drawer it had occupied since its rejection in the early 1940s. When quizzed on it by his drinking buddies, O’Brien would say that he had left it in the boot of his car, only for the manuscript to be blown away page by page on a holiday in Donegal. The Third Policeman is (among other things) a terrifying portrait of fear and loathing in the Irish Free State. The mismatch between the fantasy worlds carved out by its characters and the everyday reality of squalor and alcoholism endured by its author is among the most painful aspects of O’Brien’s life.

The collaborative nature of O’Brien’s Cruiskeen Lawn column has long been public knowledge, with Niall Montgomery stepping in whenever O’Brien was ‘unwell’ in the Jeffrey Bernard sense. It is dispiriting in the extreme, then, to see O’Brien lash out at Montgomery in 1964, though later correspondence suggests the slight was forgiven by his architect friend. He and Timothy O’Keeffe, O’Brien’s endlessly indulgent publisher, are the true heroes of this book. As we hurtle miserably towards O’Brien’s death on 1 April 1966, at the age of fifty-four, we dread each new eruption of rage almost as much as the letters’ original recipients must have done. Most of the time the later O’Brien was, quite simply, a miserable auld ballocks. Such was his hostility towards the Gardaí, who were pursuing him - unfairly, he felt - for drink-driving, that he planned to have an engineless car towed into place outside a pub with the intention of climbing into it at closing time, before triumphantly denying to the waiting constables that he was drunk and in charge of a vehicle. Woe betide the publican who served him less than a full measure of spirits (he carried his own measure, just to be sure) or attempted to close his premises before O’Brien fancied moving on. Hyperbolic self-praise of his latest projects becomes a tedious leitmotif. The endless letters about double taxation on translations of At Swim-Two-Birds are unbearable. While writing The Dalkey Archive he becomes obsessed with the idea that St Augustine may have been black, and inflicts his theories, N-word and all, on his correspondents in screaming capital letters, over and over again.

In a book full of crackpottery, one of the strangest moments comes in 1965, when O’Brien suggests that the French edition of At Swim-Two-Birds be translated back into English by a serving French Foreign Legionnaire and used to replace O’Brien’s original text, which he had come to detest. In its daft way, the suggestion is a perfect example of O’Brien’s estranged relationship with language - language in general but also the language of his artistic prime, as surveyed from the wreckage of his final years. It would be a grim but appropriate irony to describe this collection of letters as a sobering volume. Glimpses are caught of the comic genius behind the three great novels and the best of Cruiskeen Lawn, but in the main we are forced to pick our way through a disaster zone of headlong self-destruction. Might a non-drinking O’Brien have been a happier and more savoury human being? Almost certainly. Might a happier and better-adjusted O’Brien have ever written anything? Impossible to say. As it is, our reaction to the unhappy soul captured in these letters will probably be, in the words of a 1965 letter, ‘halfway between a guffaw and a puke’.