KURT VONNEGUT chain-smoked unfiltered Pall Malls from the age of 11 and was fond of saying that it was a civilised way of committing suicide. He once used pills to make an actual suicide attempt which his son Mark now says was a "bizarre, surreal incident but it never felt like he was in any danger". When Vonnegut died last year he was 84 years old and succumbed to the effects of a brain injury sustained by falling downstairs at home. With Vonnegut, things rarely happened the way they were supposed to.

Or perhaps one thing did. His legacy includes the modern classic Slaughterhouse 5, which required several converging circumstances to put it in the public eye in 1969. It required Vonnegut to have been in Dresden during and after the firebombing of that city in 1945. It required, by his own acknowledgement, 25 years to find a voice that would do that experience justice. It required a publisher, Seymour Lawrence, an independent who worked with Delacorte Press, to recognise the value of the project. And it required a receptive audience, which the anti-war, counterculture movement of the late 1960s provided.

There are some who believe, however, that Vonnegut had been stranded since 1969 railing against war in book after book, essay after essay, talk after talk, while the counterculture deserted to business, law and engineering. Vietnam, the Balkans and Iraq played on despite him, and one Bush administration followed another. He was criticised in some quarters for the resignation suggested by the repeated use of "and so it goes" in the carnage described in Slaughterhouse 5. But under the circumstances, what else would he say? He battled on anyway.

And now a fresh twist. The publication of Armageddon In Retrospect, "12 new and unpublished writings on war and peace" according to the publisher's blurb, means that Vonnegut himself is "unstuck in time". From beyond the grave, his voice rings out again and, though the pieces are supposed to be "new", most of them read as if they were old. They are certainly undated and no attempt is made to give them context.

Eight of the 12 unpublished pieces are short stories, but it is the other six that leave the biggest impression. And if that doesn't add up, it is because the first two contributions aren't included in the arithmetic. The first is an introduction by son Mark Vonnegut which offers his perspective on his father's personality and work, illuminating, for instance, how hard Kurt Vonnegut worked at appearing off the cuff, how little confidence he had and how suddenly he moved from being neglected to being offered a million-dollar book contract.

Most interesting of all are the tensions implied between father and son. Did the conflict in Iraq really break Kurt's heart "not because he gave a damn about Iraq, but because he loved America" as claimed here? If so, it will come as a disappointment to his humanist admirers. Again, Mark Vonnegut claims that Kurt "could pitch better than he could catch" and illustrates this by saying that his father "went ballistic" when Mark wrote in an article that the pessimistic Kurt "might have envied Twain and Lincoln their dead children". But then who wouldn't swing at a pitch like that?

The second "piece" is a gem and the book hasn't even officially started yet. It is a facsimile of a letter written by Kurt Vonnegut to his parents while he was still in Germany at the end of the war. In a remarkably formal account of what happened to him, Vonnegut seems to practise the repetition of phrase eventually made famous by "and so it goes". Here, it is various accounts of the terrible ways others died "but not me". On a more trivial level, there are also two semicolons employed, both wrongly. He later advised new writers to avoid semicolons because they are "transvestite hermaphrodites representing exactly nothing".

This advice concerning semicolons is repeated in the first of the official 12 pieces. It is a lecture prepared by Kurt Vonnegut, but delivered by Mark 16 days after his father's death. In it, Vonnegut looks back and forward, examining some of the injustices of American history and revealing his humanist attitude to the afterlife (there isn't one). He even references the son who is now speaking his words. A few of the old chestnuts are resurrected, on jazz as well as semicolons, and the lecture ends with a plea for the power of jokes and the comfort to be derived from having a dog during the apocalypse. His last joke is a bad one about a shit-poo dog (a cross between a shih tzu and a poodle) and his last line, "I'm out of here", wonderfully expressive.

There is a dog in the next piece, too, which is entitled Wailing Shall Be In All Streets. The dog is still leashed to a boy and both are cremated by a man with a flame thrower in the aftermath of the Dresden bombing because there is not enough labour available to keep throwing the bodies on to funeral pyres. Here the letter Vonnegut wrote to his parents is played out as a longer essay with unflinching honesty and almost unbearable horror. Vonnegut weeps for the people incinerated and for the destruction of the city itself which was "built in the art spirit, symbol of an admirable heritage, so anti-Nazi that Hitler visited it but twice during his whole reign, food and hospital center so bitterly needed now - ploughed under and salt strewn in the furrows".

AND if that was the end of Armageddon In Retrospect, the book might be regarded as a minor classic, though minor in both senses as it would only be 45 pages long. In fact, there are 10 more pieces, all of them unpublished short stories, and it is here that the reader finds himself asking the usual question of posthumously published stories - why were they not published before? After the powerful impact of the non-fiction pieces, the short stories are a disappointment. In one of them, there are soldiers obsessed with eating, which seems likely enough under the circumstances. In another, it is no surprise that a young boy would prefer playing in the carcass of a tank to lying beside an old man admiring nature. The most interesting story explores the actions of a traitor from the German-American Bund, a pro-Nazi American organisation, who is deployed as a spy among American prisoners of war.

After a while, you realise that what you are reading is practice - an author practising for greatness. These (presumably) early pieces, with their often simple didactic, are the building blocks that Vonnegut used to work towards his classic and complex anti-war novel. If they seem a bit tame, it is because he hadn't found the voice yet to make them otherwise. Tameness also afflicts the drawings by Vonnegut that illustrate the stories. There are no renderings here of anal sphincters and "wide open beaver" that had some of the citizenry averting their eyes from his Breakfast Of Champions in 1973. Instead there are simple drawings of tanks, jousting figures, skulls and crossbones, and a hirsute self-portrait.

So it is worth it? Well, yes it is fascinating to see how a craftsman perfects his craft. And if first-time readers of Vonnegut read these short stories and wonder what all the fuss was about, the non-fiction pieces will surely make them go back to find out.

Armageddon In Retrospect is published on May 15 by Jonathan Cape, priced £16.99