When Roberto Bolaño died aged 50 in 2003, he’d only witnessed the first flickers of the extraordinary fame that was to come his way. His life until that point had been hand to mouth and itinerant, divided between Chile, where he was born, Mexico City and the Costa Brava, where he worked a variety of menial jobs in the rundown seaside resort of Blanes. By the time the Anglophone world got to read him, he was months from death – New Directions published the short, hallucinatory By Night in Chile in March 2003. He died in July, two places from the top of the waiting list for a liver transplant that might have saved him.

Bolaño’s reputation as the greatest Latin American author of his generation rests on two novels – The Savage Detectives, published in Spanish in 1999 and in an immaculate English translation by Natasha Wimmer in 2007, and the book he finished on his deathbed, the Pynchonesque maximalist mindstorm that is 2666, published in English in 2008 (and also translated by Wimmer). The ambition, scope and brilliance of these books won him a host of awards, extraordinary sales (particularly given that they were novels in translation) and a cult-like following. Bolaño’s journals and interviews, pored over by his fans in the wake of his death, helped stoke the myth of his life, which in turn fed his literary reputation. Was he a freedom fighter during Pinochet’s military coup? Was he a heroin addict? To what extent were his novels of sex, drugs and avant-garde poetry romans à clefs?

What the wild success of The Savage Detectives and 2666 did was to drive demand for more Bolaño novels. Luckily enough, there was a significant body of work that had yet to be translated, much of it not even published in Spanish. And so those of us who’d fallen in love with those two reputation-establishing novels began along a strange and disheartening path of diminishing returns. The problem is that his career builds towards The Savage Detectives and 2666, and reading the work that came before it, it is as if we are following his evolution as a writer in reverse, with each new novel not quite as good as the one that preceded it, and none of them fit to share a shelf with his two masterpieces.

Was he a freedom fighter? Was he a heroin addict? Were his novels of sex, drugs and avant-garde poetry romans à clefs?

The Spirit of Science Fiction is one of Bolaño’s earliest books, written in 1984. Whereas other early novels – Woes of the True Policeman, Monsieur Pain and The Skating Rink – point towards the genre-bending plasticity of 2666, The Spirit of Science Fiction is clearly a rehearsal for the Beat-ish Künstlerroman of The Savage Detectives. It tells the story of two young, aspiring authors in Mexico City, Jan and Remo, the former a near-hermit who writes letters to the leading science-fiction authors of his day, the latter a garrulous boy on the make in a city mad about poetry. The passages about the boys and their glamorous friend José Arco are intercut with a long, dreary interview in which a successful author recounts his time at “The Potato Academy” and a series of bizarre and increasingly tedious dreams.

It’s easy to see why this novel was never published in Bolaño’s lifetime. It’s a rambling, dispiriting mess, symptomatic of the way publishers have dredged up substandard work from this great writer’s past in the hope that it might catch some of the reflected glory of his two great novels. Let us hope The Spirit of Science Fiction is the last of these tawdry outtakes that can only serve to diminish the legacy of one of the most remarkable literary voices of the past 50 years.

• The Spirit of Science Fiction by Roberto Bolaño is published by Macmillan (£14.99). To order a copy go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £15, online orders only. Phone orders min p&p of £1.99