He’s doing the promotional rounds for the latest instalment in the Discworld series, Raising Steam. It is, in short, about trains. If you are not a fan of Pratchett’s work, and know him only by reputation as a “comic fantasy” writer, then this might be surprising to you; you might expect a series of pastiches of Tolkien, with orcs and elves having battles, and magic swords and rings. Not stories about the invention of steam power, or the rise of the postal service, or newspapers and printing presses, topics covered in recent books.

In its early years, the series – set, for those of you who aren’t familiar with it, on a world which is disc-shaped (obviously), resting on the back of four gigantic elephants, which are themselves standing on an enormous interplanetary turtle – was fantasy in the classic mould; fantasy with a sharp, wild sense of humour, and a sly line in social commentary, but fantasy none the less. There were trolls and dragons, barbarian warriors and implausibly breasted Amazons in scanty armour, and wizards with staffs with knobs on the end; it took these tropes and subverted them, but that was its source material. But as the series – and its author – have matured, it has changed. The great city of Ankh-Morpork, the scene for many of the stories, has slowly turned from a Dungeons & Dragons place of rogues and thieves brawling in taverns, into a fantasy mirror of London. It has become less about magic, and more about people, and ingenuity, and technology.

“But technology is magic!” he cries, when I put this to him. “I mean, on my wrist, this will tell me the time if I ask it.” He prods a button on his watch, and it speaks in a prim little voice: the time is 11.43am, on Wednesday the so on and so forth. “And this is all magical, but we don’t think so. Well, we know it’s not magic – we know the fairies aren’t doing it, but for most of us they might as well be.”

A conversation with Pratchett is a strange experience, filled with what I can only describe as a nostalgia for the future. Raising Steam, for all its fantastical setting, is a look back to a time in earthly history when technology, he says, filled people with excitement. “In Brunel’s time, this was all wonderful stuff, it was great, the world is moving on. Oh yeah! More stuff!” he says. Now, though, he thinks, that excitement has gone. “Sometimes I get appalled at the fact that people say, ‘What’s the point in going to space? It’s of no use. Why are we spending so much money?’ and I think: ‘Bloody hell! Everyone in the world can now know where they are!’ A tiny box that lets you know exactly where you are on the planet, or another tiny box that lets you talk to anyone in the world. And we don’t think about it, we think it’s just like God gave it to us.”

Pratchett’s nostalgia is a geek’s nostalgia, a nostalgia for when you could take much of the world apart and put it back together again. “The age of steam had nuts and bolts. You knew how things were made. They were put together by engineers, like Brunel,” he says. Pratchett is particularly enthralled by trains. “Do you know about Richard Trevithick? He made a steam engine before Stephenson. And it looked pretty much like a monster. It worked, but it didn’t look anything like a train as we know it.” He gets especially excited talking about his journey on one of Britain’s surviving steam trains, the Watercress line at Alton. “I was in the engine! By the firebox. It was very hot,” he adds, a little unnecessarily.

Pratchett learnt this love of dangerous machinery and fancy gadgets from his father, growing up in Beaconsfield, a smallish town to the west of London. “My dad – you have a dad, did he have a shed?” A garage, I say. He waves this aside. It’s a shed in spirit, he says. “I got a magneto [generator] and put it up in my dad’s shed, so that when he pulled open the shed door, it electrocuted him.

“And he was very impressed. He said, ‘Well done. But don’t do it again.’ That was my dad. He would test electricity by putting his finger on it. His hands were so callused from work, because he worked on, well, everything, more or less, so he was impervious to electric shocks. Rob and I often talk about this. We both had fathers who let you play with the stuff that you shouldn’t play with.”

His mother, meanwhile, helped him with book learning. “I wasn’t particularly interested in books,” he says. “And my mum, God bless her, she rolled up her sleeves and gave me a penny per page, and it worked beautifully. I think she only gave me about thruppence, because the third book was The Wind in the Willows.” He was so enthused after this, she no longer needed to pay him. Indeed, Pratchett got a job in Beaconsfield library. “You’re talking to a man who thinks, mostly, that his school days assisted him not at all, but the library did, in spades.” He looks at me sharply. “You, when you were young, read lots of books, didn’t you? A –” he pauses, and chooses his next word carefully – “a —-load, I believe?” I did, I reassure him. “A library boy. I recognise the kind. I was the same.” He had an indifferent time at school – he grumbles about the “death or glory” nature of the 11-plus (he passed easily), and about old teachers who had a grudge against him at the High Wycombe Technical High School (“sort of half a grammar school. A big woodwork place”). But the fire kindled by Kenneth Grahame, and Ratty, Mole and Badger, grew, and blazed.

Pratchett’s own sense of humour, a sort of gentle, English, observational thing, stems from this period. “Wodehouse, obviously, but also I tore my way through the Just William books. Richmal Crompton was a very good writer. I think it was from her that I learnt irony. It took me a while to work it out.” Do you think you could define irony, I ask him. “Sort of like iron.” I deserved that, I acknowledge. “When you get hit on the head with it, you know it.”

He also fell in love with RJ Yeatman and WC Sellar, authors of 1066 and All That (“in the Thirties, when the middle classes were getting richer, the two of them really got as much fun out of that as you could. The Thirties were an awful lot of fun. Or at least until the end. Bad ending, the decade, admittedly”) and fell out with his headmaster for “bringing in a copy of Mad magazine. How horrible! And a copy of Private Eye. Seditious.” But it was the now defunct satirical magazine Punch which really formed the comic voice in which he now speaks. “I read my way through all the bound Punches. It was the best way to read history; you got it without granny looking over your shoulder, and it was just astonishing.

“And just about any writer of distinction, anywhere in the English language, worked for Punch. Mark Twain. Jerome K Jerome. And they spoke with the same voice, which opened the door for me – the same kind of slightly satirical, people-are-rather-silly-but-they’re-not-that-bad voice, friendly about humanity, fond of its foibles.” Apart from the books, the other influences of his youth are clear in his own writing – especially the later Ankh-set works, in which he frequently extols the virtues of the poor-but-respectable people living in tiny, tidy terraced houses, and of the self-made men and women. “There used to be a sort of dignity in labour,” he says. “I don’t think there is now.”

He has spoken, often, of how his time on local newspapers made him. He started at 16, in high dudgeon at his headmaster: “On my last day at the school, I left all my stuff behind and phoned up the editor of a local newspaper. He actually used some cliché like, ‘I like the cut of your jib, young man’, or something.” It is the stuff of legend that he saw his first dead body the next day, “work experience really meaning something in those days”, as he put it in his author’s bio in his books.

“Truthfully, without over-egging it, as I often do,” he says, “the library and journalism, those things made me who I am. Journalism makes you think fast. You have to speak to people in all walks of life. Especially local journalism. London journalism can p— in someone’s face and they can’t do anything about it. Try that in local journalism, and someone’s down to complain. Everyone should have one local journalism job in their lives, especially if they’re a nosy parker.” He talks of local journalists in the same way he does his parents, with a sense of quiet heroism. “I interviewed an elderly journalist who’d worked in a small town for a very, very long time. I asked: is it boring? And he said: over there, that’s where a couple pushed their daughter into the attic because she’d had a black baby. And over there, that’s where a man was caught in flagrante delicto with a barnyard fowl. And he’d said to the magistrates, ‘Well, it was my fowl’. Even those small moments, they make you realise the world is not as you thought.”

There is a shadow hanging over us during all of this, and that is Pratchett’s PCA. The disease affects, among other things, the visual centres of the brain, and the effect is quite noticeable. He struggles, for instance, to put his mojito on the table accurately; it seems as though he can’t easily tell where the tabletop begins and ends. In other interviews, though – even quite recent ones – it had seemed as though his mind was as sharp as ever.

I am desperate to report this is still the case, but I can’t. There are moments that could easily be ordinary lapses – he has to shout through to Rob a couple of times, when he forgets a word or a name, and he tells a couple of stories twice. But I start to notice, as the interview progresses, that while he is giving me fascinating answers – long, discursive stories about all sorts of things – they’re not quite answers to the questions I actually asked. For example, I learn that he didn’t know how the book Small Gods was going to end until it ended – he had the characters in place and they led him to an unavoidable conclusion.

He calls this process “narrativium” and he’s having the same experience with his next book, which he’s already started. But I’d actually asked him about the morality of one of the characters in Small Gods. It feels as though sometimes, when he can’t quite fish the answer to the question out of his brain, he substitutes a readily available anecdote on a related topic instead, one he’s rehearsed before. There is a slightly sad moment when, after one such not-quite answer, he pauses, and his voice lowers.

“You know I’ve got PCA? The other day I met up with a lot of other people with it. They were from all over the world. And it was great fun.” How so, I ask. “I’m not normally the sort of person who goes to that sort of thing. But I found you could say, ‘Don’t you like it when…’ and they’d say, ‘Oh yes’. You understand each other’s experiences.” It feels like an admission that he’s struggling with bits of the conversation in a way that someone without the condition can’t understand.

The visual problems have stopped him typing; for some years now he’s been writing his works with a combination of dictation software and a close partnership with Rob. “I keep saying, speech comes to us. We are monkeys. Speaking, chattering, is what we do,” he says, a line he’s used before. I try to ask him if he feels his writing style has changed – I’ve noticed, for instance, that the rapid back-and-forth dialogue of earlier books has been replaced, in very recent ones, with chunks of exposition, almost soliloquies. But he doesn’t quite respond, telling me instead about how hard it was getting the accent of one character on the page using the dictation software. He’s still a thoroughly engaging conversationalist, and has a wealth of stories and sharp retorts, but the impact of the PCA is unmistakable.

And, of course, it is going to get worse, and he knows it. He has said, before, that he wants to die at a time of his choosing, in his Wiltshire garden, drinking an excellent brandy and listening to his iPod. He nods. “Thomas Tallis’s Spem in Alium! That’s the one where every single part of it comes together at once, where God picks you up and drops you on your head.” Pratchett is an atheist, but he is given to religious language from time to time.

That dying moment sounds wonderful, I say. Won’t the fact you will be able to enjoy it make you think: I don’t want to leave this? “Very probably. Knowing that you can doesn’t mean that you will. The knowledge that you could stop this, if you really wanted, is predicated on you saying, ‘No, tomorrow’s going to be pretty OK, and the next day’.” The right to choose his death might let him live longer, he thinks. He made a BBC documentary, Choosing to Die, which followed a man called Peter Smedley’s decision to go to Switzerland’s Dignitas clinic to be helped to commit suicide.

“Frankly, Mr Smedley would probably be with us now, if he hadn’t had to go to Switzerland to do it. And that’s why I hate the people who try to get in the way of this,” he says, getting genuinely angry. “In Oregon [where assisted suicide is legal], you get given the potion. And often, the people who get the potion don’t use it. They die with it still in the cabinet. They were having so much fun, the grandchildren will be coming to visit and all that sort of thing, and suddenly without them thinking about it, death turned up and took them.”

I get embarrassed, at this point, asking a man about his own, presumably relatively imminent, death, and change the subject. “You’re far too nice to be a journalist,” Pratchett tells me, not unkindly. “Don’t worry about depressing stuff. I quite like depressing stuff occasionally.” But Death, as well as something Pratchett faces in the not-too-distant real-life future, is a regular character in the Discworld books: a grandfatherly figure who loves cats and curry and who speaks in _{SEPULCHRAL SMALL CAPS}. He turns up in every one of the adult novels to date, except the most recent, Snuff – and, I notice, he doesn’t make an appearance in the 150 pages of Raising Steam I’ve seen. Has he killed off Death, I wonder. “You won’t find him in the new book,” he says, chuckling but looking slightly awkward. “It’s not deliberate, but I don’t want to be a death fetishist.”