BEST-selling author Sir Terry Pratchett, who lived near Salisbury, has died aged 66 after a very public struggle with Alzheimer's disease.

His death was announced to fans on Twitter in a series of messages shortly after 3pm today.

The messages read: "AT LAST, SIR TERRY, WE MUST WALK TOGETHER.

"Terry took Death's arm and followed him through the doors and on to the black desert under the endless night.

"The End."

Matt Holland, director Swindon Festival of Literature, said: “He came to the festival in 2002 and it was interesting as we were a young festival at the time and he was the first author who really attracted people from far and wide.

“We had to get a bigger venue because the demand was so great. He was a small man in staure but big in ideas. Fantasty isn’t necessarily my cup of tea but it was great to experience his terrific sense of ideas and irony.

“He was a lovely guy and I think his work will live on as it is very intelligent writing. I believe it will out last the Harry Potter stuff.” 

Sir Terry's publisher, Larry Finlay, said he was "deeply saddened" by the loss of one of the world's "brightest, sharpest minds".

Mr Finlay, managing director at Transworld Publishers, said he "passed away in his home, with his cat sleeping on his bed, surrounded by his family".

He completed his last book - set like so many of his best-sellers in Discworld - last year.

Mr Finlay said: "In over 70 books, Terry enriched the planet like few before him. As all who read him know, Discworld was his vehicle to satirise this world; he did so brilliantly, with great skill, enormous humour and constant invention.

"Terry faced his Alzheimer's disease (an 'embuggerance', as he called it) publicly and bravely. Over the last few years, it was his writing that sustained him. His legacy will endure for decades to come.

"My sympathies go out to Terry's wife Lyn, their daughter Rhianna, to his close friend Rob Wilkins, and to all closest to him."

His comic fantasy tales set in Discworld - which was a flat disc balanced on the backs of four elephants stood on the back of a giant turtle - made millions laugh and made them think as well.

His sense of fun made him stand out in the often po-faced world of fantasy literature - he would often turn up at conventions wearing a T-shirt saying: "Tolkien's dead, JK Rowling said no, Philip Pullman couldn't make it. Hi. I'm Terry Pratchett."

Towards the end of his life, he used his fame and wealth to campaign for a greater awareness of dementia and assisted dying.

In 2011, he featured in a documentary about suicide in which he followed a man with motor neurone disease to the Dignitas clinic in Switzerland to see him take a lethal dose of barbiturates.

Asked why he wanted to make the film, he said it was because he was "appalled" at the state of the law.

A year earlier, he had used the prestigious Richard Dimbleby Lecture to call for assisted suicide to be legalised.

Hilary Evans, director of Alzheimer's Research UK, said the death of Sir Terry would have "a profound effect on both literature and the 850,000 people who live with dementia".

She said his announcement of his own diagnosis was "a watershed moment" and "a call to arms for society to talk about dementia and take steps towards defeating it".

Prime Minister David Cameron said: "Sad to hear of Sir Terry Pratchett's death, his books fired the imagination of millions and he fearlessly campaigned for dementia awareness."

Sarah Wootton, chief executive of Dignity in Dying, called him "a committed campaigner" for "assisted dying".

She said: "Sir Terry was fond of saying 'It's time we learned to be as good at dying as we are at living' and his brave approach to confronting issues of death, including his own, was a heartfelt demonstration of dignity."

Alzheimer's Society chief executive Jeremy Hughes said Sir Terry had "fundamentally changed the way dementia is seen and understood" and that he was "the most passionate of campaigners to bring change" who was determined to reduce the stigma of dementia.

Thinking of how he will remember Sir Terry, he said "shouting from the rooftops about the absurdity of how little funding dementia research receives, and fighting for good quality dementia care, he was and will remain the truest of champions for people with the condition".

Sir Terry started his writing career as a reporter for the Bucks Free Press and also joined the Western Daily Press and the Bath Chronicle before becoming a press officer for the Central Electricity Generating Board, with responsibility for three nuclear power stations, in 1980.

He published his first novel, The Carpet People, in 1971 but his career as a novelist really took off after the publication of the first Discworld book, The Colour Of Magic, in 1983.

His books sold millions of copies worldwide and were translated into more than 30 languages.

Asked about his success in 2009 when he was knighted, he said: "Most writers don't make much money, they only happen to make some if they're standing in the station when the gravy train comes in.

''I thought I was lucky to make some money, then lucky to make a living, then lucky to be a millionaire.''

Sir Terry's daughter Rhianna, announced his death on Twitter in the style of his own writing.

She wrote: "AT LAST, SIR TERRY, WE MUST WALK TOGETHER. Terry took Death's arm and followed him through the doors and on to the black desert under the endless night. The End."

She later wrote: "Many thanks for all the kind words about my dad. Those last few tweets were sent with shaking hands and tear-filled eyes."

A JustGiving page in memory of the writer raised thousands of pounds for an Alzheimer's charity in his memory less than an hour after his death was announced.