Two deaths made this book possible. First, the fatal stabbing of David Kammerer in 1944, around which this novella revolves like a ghoulish carousel; then the passing in 2005 of Kammerer's attacker, Lucien Carr, which meant publication could finally go ahead.

Burroughs and Kerouac were 30 and 21 respectively when they composed And the Hippos Were Boiled in Their Tanks (the surreal title alludes to a report of a fire at a zoo), writing alternate chapters under pseudonyms. Carr and Kammerer were their friends, and the events surrounding the latter's death were still fresh in their minds. A bloodstained Carr sought out both men straight after the attack, but whereas the worldly-wise Burroughs calmly advised Carr to turn himself in, Kerouac buckled under the strain. "My legs kept bending at the knee," he says, as Carr confesses all in a bar.

Nevertheless, Kerouac can barely disguise his excitement at this unexpected exposure to a real-life drama. "I used to imagine what it would be like to kill someone," he admits in a key passage. "Now here stood Phillip [Carr] beside me, and he had actually done it." Hippos shows that the Beats' genius for self-mythologising, their unwavering belief in themselves as existential heroes rather than aimless losers, set in early.

Without Carr, however, the whole Beat phenomenon might never have happened, for he not only introduced Kerouac, Burroughs and Allen Ginsberg to one another, he also inspired them by setting a bad example. Carr's relationship with Kammerer (14 years his senior) is reminiscent of Joe Orton's with Kenneth Halliwell, the older man introducing his protégé to literature, then panicking when it seemed he might leave. Finally, forcing his attentions on Carr in a park on New York's Upper West Side, Kammerer was rewarded with a pocket knife in the heart.

Carr served two years for manslaughter, then tried to put his past behind him, persuading his friends never to publish Hippos in his lifetime. Not that they could find a publisher. "It had no commercial possibilities," Burroughs later explained. "It wasn't sensational enough to make it . . . nor was it well-written or interesting enough to make it [from] a purely literary point of view." This verdict still stands.

Neither Burroughs nor Kerouac is at his best here, but Hippos has value as a testament to their latent talent. Both men, though young, come across as natural writers with an instinct for the telling detail. Burroughs is grimly fascinated by the abuse of authority, his sarcastic, petty-minded landlord Mr Goldstein being a distant relative of the County Clerk in Naked Lunch. If anything, Hippos proves that becoming a junkie was the making of Burroughs, pulling his unique vision into focus.

Kerouac's best set-piece is his description of the bizarre day he spent with Carr - watching The Four Feathers and standing in silent contemplation of Modigliani's portrait of Cocteau at MoMA - before they parted and Carr confessed to the police. Kerouac also brings alive the exotic, homoerotic allure of the waterfront and a life at sea, as he and Carr plan to sign on as seamen and reach Paris in time for the liberation. "Everyone who has lived through a war, any sort of war, knows that beat means, not so much weariness, as rawness of the nerves," said John Clellon Holmes, trying to define the Beat generation in the late 1950s. Hippos, with wartime New York as its setting, has that sustained, nervous tension and sense of impending doom.

In the summer of 1944 Jack Kerouac was 23, William Burroughs 30. Both were members of a loose New York coterie of artsy dropouts but neither had written anything substantial. Drifting between late-night bars, friends' apartments and union halls, they were slowly becoming themselves, Kerouac slumming around as a half-hearted merchant seaman and Burroughs embarking on his patient metamorphosis into a wire-veined votary of morphine. In those early days the Beat generation hadn't even realised it was beat. It was just broke and bored. And then, something happened.

David Kammerer, a school friend of Burroughs, had come to New York to pursue his infatuation with Lucien Carr, a beautiful boy from a wealthy family. Years earlier, Kammerer had been Carr's Boy Scout leader in St Louis. The two washed up in Manhattan, where on a mid-August night they shared a bottle of wine in Riverside Park. By the time the sun came up, Carr had stabbed Kammerer in the chest with his scout knife and sunk the corpse in the Hudson river.

The crime was front-page news. It had everything: the rich boy, the cliché of the predatory pederast, the setting in decadent urban bohemia. Burroughs and Kerouac were arrested when it emerged that Carr had gone to them afterwards for advice (Kerouac even spent the following day helping to dispose of the murder weapon and persuading Carr to turn himself in). Once Carr was in prison, they decided to collaborate on a true-crime novel. They wrote alternate chapters, adding a suitably frenzied title from a news report about a circus fire.

And the Hippos Were Boiled In Their Tanks is now published for the first time. A barely encrypted roman à clef, it is narrated by Will Dennison (Burroughs) and Mike Ryko (Kerouac), with casting couch stand-ins for Carr and Kammerer: 'Phillip Tourian is 17 years old, half Turkish and half American [...] the kind of boy literary fags write sonnets to [...] Ramsay Allen is so stuck on Phillip he is hovering over him like a shy vulture, with a foolish sloppy grin on his face.'

The book is unmistakably a tyro effort but Kerouac thought it rather good then, writing to his sister about a 'portrait of the "lost" segment of our generation, hard-boiled, honest, and sensationally real'. Burroughs later described it as 'not a very distinguished work'. He was right. The obligation to change names and facts also ruins at least one narrative opportunity: the tragic detail of the scout knife. Here, Tourian kills Allen with a hatchet, a rare example of fact plotted more neatly than fiction.

For Beat aficionados, however, there are rough gems. Burroughs describes a morphine kick in a passage that could be injected uncut into Junky; and, after saying goodbye to Phillip, 'Mike Ryko' takes the first staggering steps towards On the Road, deciding 'right then and there to go off and travel again. I felt like seeing the Pennsylvania hills again, and the scrub pines of North Carolina.'

These are the highs. But the most successful thing in Hippos is its low, sustained growl of violence. Burroughs has sailors brawling and policemen beating up harmless drunks; Kerouac's merchant marines go on an orgiastic rampage in Nova Scotia long before the mysterious unseen killing at the book's climax. And it's Kerouac who points to the wider violence boiling away far from New York's melting pot: 'In the slip a barge was docked, alongside another Liberty ship, and a tremendous crane was hauling up 20mm anti-aircraft guns to the platform on the flying bridge of the ship. Phil and I watched some of this for a while, then we picked up our bags and left.'

This is numb, declarative writing. Right up to the late, underrated Vanity of Duluoz, Kerouac kept telling the Carr/Kammerer story. What shocked him was the sheer absence of shock, 'as tho someone'd woken me up with the news of a new leak in the cellar'. Hippos, rough as it is, catches something of that anomie - but it also raises the possibility of escape. Burroughs chose heroin, and never got free of the addiction. Kerouac went west; Carr served two years in jail before becoming an editor. The war ended, and the Beats went on.