He had been living with Alzheimer's for years.
Sir Terry Pratchett obituary
Author Terry Pratchett Was No Stranger To Death : NPR
RIP Sir Terry Pratchett
Sir Terry Pratchett dies, aged 66: the tributes as they happened
Beloved Fantasy Author Terry Pratchett Has Passed Away at Age 66 - Electric Literature
R.I.P Sir Terry Pratchett
Terry Pratchett, Tolkien fan and fantasy author, has passed away – The Tolkien Society
Fantasy Author Sir Terry Pratchett Dead at 66
Terry Pratchett, fantasy author; at 66 - The Boston Globe