Writer Sir Terry Pratchett has died at the age of 66, his publishers said.
Larry Finlay, managing director at Transworld Publishers, said he was "deeply saddened" by the news.
He said: "The world has lost one of its brightest, sharpest minds."
Sir Terry, who wrote more than 70 best-selling novels, waged a very public struggle with Alzheimer's disease in recent years.
His publisher 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 his creation of 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."
Sir Terry's thousands of fans on Twitter were alerted to the news by a series of messages shortly after 3pm.
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."
Four years ago, Sir Terry 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.
He said: "The Government here has always turned its back on it and I was ashamed that British people had to drag themselves to Switzerland, at considerable cost, in order to get the services that they were hoping for.''
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".
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."