Fantasy author Sir Terry Pratchett has died aged 66, having had Alzheimer's disease for eight years.
"The world has lost one of its brightest, sharpest minds," said Larry Finlay of his publishing company, Transworld.
Best known for the Discworld series, Sir Terry wrote more than 70 books over his lengthy career.
He was first diagnosed with Alzheimer's in 2007, but continued writing, completing his final book last summer.
The author died at home "with his cat sleeping on his bed, surrounded by his family," Mr Finlay said.
"In over 70 books, Terry enriched the planet like few before him," he added.
"As all who read him know, Discworld was his vehicle to satirize 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."
The Discworld series - which started in 1983 - was based in a flat world perched on the backs of four elephants which, in turn, stand on the back of a giant turtle. By 2013, he had written more than 40 instalments.
At the peak of his writing powers, Sir Terry was publishing more than three books a year. His quirky and satirical view of the world won him a worldwide following.
At the turn of the century, he was Britain's second most-read author, beaten only by JK Rowling.
Known for his striking dress sense and large black fedora, Sir Terry was awarded an OBE for services to literature in 1998.
In August, 2007, it was reported he had suffered a stroke, but the following December he announced that he had been diagnosed with a very rare form of early-onset Alzheimer's disease which, he said, "lay behind this year's phantom stroke".
Knighted in 2009, he said: "It would appear to me that me getting up and saying 'I've got Alzheimer's', it did shake people."
"The thing about Alzheimer's is there are few families that haven't been touched by the disease.
"People come up to me and talk about it and burst into tears; there's far more awareness about it and that was really what I hoped was going to happen."
His death was announced on his Twitter account, on Thursday afternoon.
The first tweet was composed in capital letters - which was how the author portrayed the character of Death in his novels.
"AT LAST, SIR TERRY, WE MUST WALK TOGETHER," it stated.
"Terry took Death's arm and followed him through the doors and on to the black desert under the endless night."