After he was diagnosed with Alzheimer’s, the Discworld author began an autobiography. He never finished it, but seven years after his death, his long-time assistant has taken up the task

Five months before he died, Terry Pratchett wrote five letters, sealed them in envelopes and locked them in the safe in his office to be opened after his death. This was the one he addressed to me.

So. I have gone. There were days when I felt I had already gone and so all I wish for now is a cool, quiet room and some peace to gather my addled thoughts. I think I was good, although I could have been better, but Terry Pratchett is dead and there are no more words.

Look after Lyn, please. Have those fine pieces of jewellery cast to my design and give them with my love. Choose a gift every Christmas and birthday. Send flowers. Have a big dinner each year, more if necessary or if a celebration is required, and raise a brandy to my memory and to happy days.

Look after the business and it will look after you. For all you have done, for all of the little things and all of the much bigger things and for the burying of the bodies … I thank you.

Learn to fly. Do it now.

And mind how you go.

Just to be clear: there were no actual bodies in need of burying during my more than 20 years of working with Terry Pratchett. Terry could get quite exasperated with people sometimes, and certainly did not (as people often found themselves saying about him) suffer fools gladly. But he never got that exasperated.

After Terry was diagnosed with Posterior Cortical Atrophy, a rare form of Alzheimer’s disease, in 2007, at the cruelly early age of 59, I began to accompany him at public appearances, reading for him when he no longer could, helping him through interviews on stage as “keeper of the anecdote”. We became, of necessity, a sort of double act.

There were, inevitably, grim and testing times in those years, and I spent a lot of that period in denial about the full gravity of what was unfolding. Yet Terry was doing exactly the opposite, reacting to the news of his imminent demise with bravery, with unsparing thought, with a determination to confront his condition head on in public, with a bold mission to force the topic of assisted dying into the national conversation, and most of all (being Terry) with work – three television documentaries and seven more bestsellers.

Terry often talked about “doing” his autobiography. In the years before he was ill, he talked about it almost exclusively to dismiss the idea. He didn’t seem persuaded that there was anything in the story of the journey that took a kid from a council house in Beaconsfield to a knighthood and a mansion near Salisbury by the sheer power of his imagination alone; or in the tale of how a boy with, as Terry put it, “a mouthful of speech impediments” became one of his generation’s most popular communicators; or how someone who left school with five O-levels could also go on to have an honorary professorship at Trinity College Dublin. And besides, there were always other things waiting to be written – bigger stories in which far more outlandish and arresting things were free to happen.

But now that Terry’s memory itself was under an explicit threat, the prospect of a memoir felt different. Even in the car driving back together from Addenbrooke’s hospital in Cambridge that awful December afternoon when the devastating diagnosis had been given to him, Terry started talking about his autobiography – about how he needed to get going on it, and how the clock was running.

Yet we had no clear idea how long we had. One year? Two years? We had more time than we knew, in fact; it would be seven years before Terry’s last day at work. Yet, when it came down to it, the priority was always the novels – first Nation, the book Terry was working on at the time of the diagnosis, and then Unseen Academicals, I Shall Wear Midnight, Snuff, Dodger, Raising Steam, The Shepherd’s Crown … All through this period he was chasing to get those stories down.

However, there would be days, when the mood was right, when Terry would tell me to open the memoir file, and he would do an afternoon on the autobiography, him dictating, me typing. At the point at which we ran out of time, the file had grown to just over 24,000 words, rough-hewn, disjointed, awaiting the essential polish that Terry would never be in a position to give them. He was intending to call the book A Life With Footnotes.

And so here is Terry Pratchett one night in 2006, wearing a purple velvet dinner jacket with black silk lapels and sitting in a chauffeur-driven silver Mercedes as it passes below a bright electric hoarding reading: “TERRY PRATCHETT’S HOGFATHER”.

Here’s the former press officer of the Central Electricity Generating Board, South Western Region, with his name in lights – Terry Pratchett at the peak of his powers.

And here he is, as the car pulls up outside the Curzon Cinema in Mayfair, putting on his signature black fedora and gathering up his purely decorative ebony cane with its silver Death’s Head handle, and stepping out of the limo into a blizzard of flashbulbs and a gale of shouting photographers and hollering fans who are jammed along the pavement.

And then here’s Terry Pratchett the morning after, back at his desk, where the previous night is of no interest, because it happened yesterday, and because the focus must always come back eventually to a man in his gardening clothes, sitting at a screen, getting on with the next book.

“Your reward for doing something good,” Terry has taken to saying, “is to do something else good.”

He left it a bit late, of course, and he would think about that ruefully near the end, when time was running out and we were losing him at 100mph.

In an interview in 2010, Terry was asked what advice he would give his younger self.

“Get more sex while you can,” Terry straight away replied.

But then he thought about it more seriously. “I wish I had started writing for a living earlier,” he said eventually. “I could probably have started to write full time about 10 years before I did.”

What would those 10 extra years have amounted to, in Pratchett terms? Another 20 books?

In 1987, having finally taken the plunge and left the CEGB for Gaze Cottage in Somerset and “the cold waters of self-employment”, he needed structure – the structure of an office day, only stricter. He sequestered himself in his room, with only the occasional cat for company. There were to be no interruptions. Daily word targets became even more important to him, 3,000 the goal he now set himself. Terry seems to have decided that his approach to the business of being a novelist would be utterly blue-collar – industrial, even. Furthermore, he would be adamant about prioritising that industry in the face of all other claims on his time.

Aged nine or 10, his daughter Rhianna drew a picture of a hat and wrote underneath it: “I love my father but he is very busy.”

He was busy writing, but also busy building a following, at book signings, SF and fantasy conventions, games fairs – growing his brand, we would now say. The hat helped. It had been bought, in early 1988, at Lock & Company in St James’s Street, in a rare moment of extravagance. Lock & Company’s 18th-century shop, with its dark green paintwork, was itself a lasting fascination for Terry, not to mention a location to adapt eventually for the book Dodger. This single item gave him, with almost absurd ease, an image. It possessed a transformative magic, in the sense that, simply by putting it on, he could become the public Terry Pratchett that he was increasingly being asked to be. And of course, by the equally simple act of removing it, he could become himself again. It was, as he used to say, “an anti-disguise”.

It was a period of practically incessant work, but it was apparent very quickly, and very gratifyingly, that it was paying off. One Sunday, with Terry on a rare day off, Lyn opened the newspaper at the page with the bestseller lists and immediately went out into the garden to find him. “Terry, you’re number two!” she said.

Terry let it sink in for a second, before typically putting a dent in the glory of his own moment.

“Who is number one?” he said.

“Yes,” said Terry, “and I bet he’s not in his back garden fixing a puncture on his daughter’s bike.”

It all got very big, very quickly. These were the years when Terry’s career caught fire and properly blazed and when all the crucial numbers began to escalate vertiginously. Through the 1990s, Terry sold an average of 3 million books each year. Nobody in Britain sold more and, as the newspaper profiles liked to express it, if you set end-to-end every Terry Pratchett book ever bought, they would reach … well, from wherever you were to a very long way away.

Inevitably, Terry’s advances grew, too. They went from £51,000 a book to £200,000 a book, and then to £400,000 a book. And they would have carried on growing if they hadn’t met resistance from an unlikely source: Terry himself. After the six-book Gollancz deal which had floated him away from the safe harbour of full-time employment and which ended with Witches Abroad in 1991, he decided he no longer wanted the pressures of such a long-term arrangement, the responsibility of which seemed in practice to worry him more than make him feel secure. He instructed his agent Colin Smythe to strike deals for no more than two books at a time.

Terry also had strong and, some might even say, puritanical ideas about how much money he should accept in advance of a book’s publication. If he couldn’t be confident that the advance would earn itself out inside three years and that the book would go into profit and yield royalties, he refused to accept it. At one point, for example, Transworld offered Terry £125,000 for a book. This was in the mid-1990s, when a generous offer for a book of its nature would have been in the region of £25,000, so that six-figure offer was an emphatic demonstration of confidence in Terry’s writing. Colin, naturally, was excited to tell Terry about it. The conversation they had was short and pointed. Colin then found himself ringing Transworld and saying: “I have conveyed your offer to Terry, and I’m afraid he is not at all happy with it … No, he says it’s far too much and he would like me to agree a deal with you for less.”

Similar qualms on Terry’s part affected the price paid up front for Good Omens, his 1990 collaboration with Neil Gaiman. During 1985, Neil had shown Terry a file containing 5,282 words exploring a scenario in which Richmal Crompton’s William Brown had somehow become the Antichrist. Terry loved it, and the concept stayed in his mind. A couple of years later, he rang Neil to ask him if he had done any more work on it. Neil, who had been spending that time thinking about his series The Sandman, for DC Comics, said he hadn’t really given it another thought. Terry said: “Well, I know what happens next, so either you can sell me the idea or we can write it together.” Neil knew straight away which of those options he preferred. As he said: “It was like Michelangelo ringing up and saying, ‘Do you fancy doing a ceiling?’”

So, mostly in the spirit of experiment, the two of them started building a book together. It was a lark, really – a side project with nothing hingeing on it except their own diversion. According to Terry they were “two guys who didn’t have anything to lose by having fun”. They were also two guys who operated at different ends of the day. Neil, at this point in his life, was largely allergic to the morning and would wake around lunchtime to flurries of crisp answerphone messages from his collaborator, which were generally variations on the theme of “Get up, you lazy bastard”.

Yet somehow, a book emerged, in which the angel Aziraphale and the demon Crowley got together to head off the end times, and in which lastingly important thoughts were set down about witches, prophecies, the Four Horsemen of the Apocalypse and pets. The pair handed it over to their respective agents to see if it could be sold. Whereupon, this being a genuinely funny book with a manifest commercial appeal, the bidding rose rapidly into six figures and, according to Colin, would have happily continued rising if Terry hadn’t panicked and called a stop to it.

There it was again – that anxiety about being paid too much, getting caught out and thereby destroying his good name for ever. And it never left him. In 2006, I was with Terry when the offer came in for a collection of his nonfiction writing – the book which eventually became A Slip of the Keyboard. The sum was £750,000. Terry was appalled. These publishers were all mad, flinging money around. “It’s just testosterone,” he exclaimed, in high dudgeon. “I withdraw the book.” The book remained withdrawn for eight years.

It was this fear that drove him to put up on the wall of his office a large picture of WH Smith’s book-pulping machine. It was there, he said, to remind him to write a better book.

It wasn’t that Terry was squeamish about having large sums of money connected with his name. On the contrary. “Thank you for all the words,” fans would say at the signings. “Thank you for all the money,” Terry would reply. He was, perhaps, eternally a working-class boy made good. Why go to the trouble of being rich if you aren’t going to be proud of it? But he had an equally entrenched working-class belief that money had to be earned. Otherwise, what was there to be proud of? He would say: “It’s not about the money, but it’s all about the money.” And what he meant was, in a world which did not seem overly inclined to reward him with critical praise or mainstream prizes, the money was “a way of keeping score”. But, precisely because of that, he needed the score to be accurate – not distorted by news-grabbing and ultimately unworkable advances, but genuinely and calculatedly reflecting his status as a seller of books.

The truth is that very few novelists have interpreted the term “full-time” in the expression “full-time writer” as literally as Terry Pratchett did when he quit the day job. Sometimes, in that first decade, there was so much work going on, and so little time in his week for anything else, that it would even enrage him. At such moments, he would lash out at the forces that were relentlessly cracking the whip – forgetting, of course, that chief among those forces was himself.

“He once phoned me up in exasperation that he was being totally taken for granted by his publishers,” his friend Dave Busby told me. “He was fuming. He had had enough. He was going to take a sabbatical. No more writing for at least six months. I felt very pleased for him. He needed that break. I think he planned to do a lot of travelling. I did not hear from him for about six months and when we made contact again, I asked him what he had done in his sabbatical. He replied, irritably, ‘I wrote two books.’”

The plainest – and saddest – truth of all, is that his autobiography has to take its place on the long list of books by Terry Pratchett that a merciless degenerative brain disorder harshly denied us the opportunity to read. That was the loss for which those of us who loved Terry ended up grieving, on top of our grief at the loss of Terry himself, and there is simply no mending either of those gaps.

This is an edited extract from Terry Pratchett: A Life with Footnotes by Rob Wilkins, published by Doubleday. To support The Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.