WHEN Stuart Shorter, a homeless man, stepped in front of the 11.15am London to King’s Lynn train, his biographer, Alexander Masters, did not believe that it was suicide. Stuart’s death, he considered, was rather like his life — haphazard, erratic and disastrous, but not deliberate.

If Masters had never met Stuart, his extraordinary, shocking life would have remained anonymous. But Stuart, whom Masters describes as a “thief, hostage-taker, psycho and sociopathic street raconteur”, lives on in Stuart, A Life Backwards, Masters’ acclaimed biography of his homeless friend which has been shortlisted for the BBC Four Samuel Johnson Prize for Non-Fiction (the winner will be announced on June 14).

The book has been a surprise success. Publishers bid for it, and Fourth Estate finally nailed it for an undisclosed fee. There are five film offers on the table. There have been photo shoots at Vogue and excerpts published in The Guardian, The Daily Telegraph and Socialist Worker.

Stuart, with his squalid habits, his quirky delivery and his terrifying, marginal existence, embodies a bewildering slice of society that most of us know little about. He is by turns funny and sickening. The book records, with bitter humour, the collision of two wildly different social worlds, the ones inhabited by an educated, middle-class writer and his foul-mouthed, down-and-out subject. It is the result of four and a half years’ work by Masters, a former neighbour of mine, to record a life lived on the fringes of society.

Masters was a writer and illustrator who met Stuart through his work for a homeless charity. Stuart was then an alcoholic and polydrug addict who occupied the bottom rung of destitute society — known as the “chaotic” homeless. He had lived in skips and car parks, and was feared by other beggars as “that mad bastard on level D”. He had a long criminal record, a fondness for knives and suffered from delusional paranoia.

What made Masters decide to write a book about such a deeply unappealing subject? He agrees that it was an unpopular topic, and one that he found initially hard to sell to publishers, but his curiosity about Stuart’s life, and how he had ended up this way, drove him to write it.

“I would start talking to people about it and had hardly got the word “homeless” out of my mouth when their faces would flop. But I thought, ‘If Stuart can say it, if he can live it, then I can write it’.

“The problem with homeless people is this tendency to categorise them. The hard part for me was to acquire modesty. After all, here was I, very well educated with everything that (Stuart) didn’t have. When I was talking to him, I offered judgments every couple of sentences, which quite reasonably irritated the hell out of Stuart. I had to lose that approach. Drop the lecturing and listen.

“It was instructive. I realised early on that Stuart was a brilliant speaker. He had an engaging, attentive way with people. It was difficult to pace myself against him. He was constantly changing — amazingly inconsistent. At first I thought that he was lying or stupid. Now I realise that an existence as bizarre as his does not have one interpretation.”

Part of the problem with the way that we view the homeless is that we veer between condemnation of the way they live their lives and pity for them, which is simply patronising, says Masters. “The homeless are a hated group, and they’re also the object of misplaced compassion. Stuart loathed that. He felt that it ignored half of him. If someone spat at you, you’d feel annoyed. You want your revenge. You don’t want to know that they’ve been fiddled with as a child. This is insulting, and I think it is wrong to treat the homeless with kid gloves.”

So how does a writer convey the life of a homeless man? Masters began by tape-recording interviews with Stuart, but abandoned this method when he realised that his subject — who spoke in a rambling, repetitive, senseless way — really came to life in company, when he loved to tell entertaining stories and could be surprisingly incisive.

So Masters changed his tack, put himself into the narrative and the book began to write itself, becoming a story of a friendship between two mismatched men, by turns friends and sparring partners.

“Stuart was someone I found refreshing to be with,” says Alexander. “I liked and admired and trusted him, and he made me think twice about myself. He was eloquent, clear-thinking and often the first to notice other people and their tensions; he was receptive to their moods. I was amazed to find that I was often wrong-footed by this man who lived on the streets.”

Stuart emerges from the book as a man who, along with the disgusting personal habits and violent episodes, possesses magnetism and charm. There is an endearing jaunty naivety about this ruffian, as he takes tea in the country and entertains a group of Alexander’s academic friends — who are fascinated by this bizarre man — with his long-winded yarns.

But Stuart’s life is full of demons. His paranoid episodes are usually drug-induced — he sees a 6ft beetle climbing out of a bucket. At one point he is certain that the police have teamed up with his girlfriend and the ventilation repair man to record him in compromising conversations. And that a council official who had come to visit him the previous week was really planting heroin.

Much of what Masters records in Stuart’s life is a journey through Hell, explicitly documenting the horrors of low-life suffering. On the streets Stuart points out a man paralysed from the chest down after injecting himself with citric acid. “In the worst cases such a person is hardly human at all but like the shell of a man walking around crammed with minced ego. It is as though some piece of their soul is missing,” says Masters.

But mostly, all the author can do is look on like Dante, pityingly, at the stretched-out agonies before him and ask his guide what on earth got them into this state in the first place.

Masters was shocked when he heard of Stuart’s violent death so near the publication of the book. But he had sensed it. “I saw a note I had written some time ago which read, ‘I don’t think Stuart will be with me much longer’.

“I had promised him half the proceeds — it seemed the least I could do for cataloguing his life. But he isn’t around to collect it.”

Stuart, A Life Backwards by Alexander Masters (Fourth Estate, £12.99)