LONDON — Fantasy writer Terry Pratchett, creator of the exuberant, satirical "Discworld" series and author of more than 70 books, has died. He was 66.
Pratchett, who suffered from a rare form of early onset Alzheimer's disease, had earned wide respect in Britain and beyond with his dignified campaign for the right of critically ill patients to choose assisted suicide.
Transworld Publishers said Pratchett died Thursday at his home, "with his cat sleeping on his bed surrounded by his family." The firm said he died of natural causes, from a chest infection combined with the worsening effects of his dementia.
Transworld's managing director Larry Finlay said "the world has lost one of its brightest, sharpest minds."
Prime Minister David Cameron said "his books fired the imagination of millions and he fearlessly campaigned for dementia awareness."
Pratchett's death was also announced on his Twitter account, with a series of tweets that began: "AT LAST, SIR TERRY, WE MUST WALK TOGETHER."
It continued: "Terry took Death's arm and followed him through the doors and on to the black desert under the endless night."
Pratchett's ability to write and speak had deteriorated in recent years as the disease progressed. But with his characteristic black fedora and neatly trimmed white beard he remained a familiar figure in the public eye.
Transworld said Pratchett completed his final book, "The Shepherd's Crown," in the summer of 2014. It is due to be published later this year.
Pratchett was best known for "Discworld," a series of more than 40 comic novels set in a teeming fantasy world.
The "Discworld" series began in 1983 with the publication of "The Colour of Magic" and became more popular over time as the imaginary world Pratchett created became more detailed and complex.
"Discworld" grew into a series of 40 interconnected books — set on a giant disc balanced on the back of four elephants — that mixed folklore, and mischievous fun into an ebullient saga that satirized both the fantasy genre and real-life power and politics.
He sold more than 65 million books worldwide, and his novels have been translated into several dozen languages.
During the 1990s, he was Britain's best-selling author — eventually surpassed by J.K. Rowling.
Pratchett disclosed his medical condition in 2007. His doctors at first believed he had suffered a stroke, but found him to have an unusual form of Alzheimer's
He tried to be optimistic with his millions of fans, assuring them on his website that the condition didn't seem to be immediately life-threatening.
"Frankly, I would prefer it if people kept things cheerful," he said.
As he lost the ability to write on a computer, he turned to a dictation system that allowed him to keep producing fictional works, his agent Colin Smythe said.
"It may have changed his prose style slightly," Smythe said. "The real problem is the difficulty of revising it."
Pratchett didn't shy away from the emotional public debate about assisted suicide.
He used the prestigious Richard Dimbleby lecture in February 2010 to argue the logic of allowing people to end their lives at a time they chose. He said assisted suicide should be decriminalized and that suicide panels should be set up to judge cases, and offered his own case as an example.
In the lecture, Pratchett said there was no reason to believe a cure for his disease was imminent. He said he could live his remaining years more fully if he knew he would be allowed to end his life before the disease claimed him.
"I have vowed that rather than let Alzheimer's take me, I would take it," he said. "I would live my life as ever to the full and die, before the disease mounted its last attack, in my own home, in a chair on the lawn, with a brandy in my hand to wash down whatever modern version of the Brompton Cocktail some helpful medic could supply. And with Thomas Tallis on my iPod, I would shake hands with death."
Sarah Wootton, chief executive of the campaign group Dignity in Dying, said he was a committed campaigner for the cause.
"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," she said.
Pratchett was born and raised in the town of Beaconsfield, northwest of London. He described himself as a nondescript student who attended a technical high school because he felt woodwork would be more interesting than Latin. He was also interested in radios and computers.
His first short story, "The Hades Business," was published in a school magazine when he was 13 and was published commercially two years later. He used his proceeds to buy a typewriter and wrote regularly for the rest of the life, turning to journalism and writing novels in his spare time until the success of his fictional works allowed him to concentrate on them full time.
Pratchett married Lyn Purves in 1968 and published his first novel, "The Carpet People," three years later in 1971.
As well as the "Discworld" books, Pratchett also published a series of well-regarded, award-winning novels aimed at young readers.
He received a knighthood from Queen Elizabeth II at Buckingham Palace in 2009 for his services to literature. He also contributed $1 million to Alzheimer's disease research and urged the scientific community to make it a higher priority.
Pratchett is survived by his wife and their daughter, Rhianna.
Associated Press writer Jill Lawless contributed to this report.