.... have posterior cortical atrophy or PCA. They say, rather ingenuously, that if you have Alzheimer's it's the best form of Alzheimer's to have. This is a moot point, but what it does do, while gradually robbing you of memory, visual acuity and other things you didn't know you had until you miss them, is leave you more or less as fluent and coherent as you always have been.
I spoke to a fellow sufferer recently (or as I prefer to say, ‘a person who is thoroughly annoyed with the fact they have dementia’) who talked in the tones of a university lecturer and in every respect was quite capable of taking part in an animated conversation. Nevertheless, he could not see the teacup in front of him. His eyes knew that the cup was there; his brain was not passing along the information. This disease slips you away a little bit at a time and lets you watch it happen...
I suspect Pratchett knows it's likely too late for him, but this is something he can do (he also donated $1 million, he's an honorably rich man).
I've read over 45 of Pratchett's books, he's written 47. I think I've missed one or two of his very earliest, before he was famous. If this were a just world, he'd be considered for the literature Nobel. It's not too late.
Pratchett writes "fantasy" for the same reason Banks writes science fiction. It's a way to write about subjects too big for conventional literature. Yes, he also likes to entertain.
Wikipedia has a full list of his Discworld novels. You can start just about anywhere, though the later novels do expect that you've at least red the Discworld wikipedia page. My favorites are between 1990 and 1998. Small Gods (1992) and Carpe Jugulum are a good pair, and this time of year Hogfather is a family favorite. All of the books are available through the St. Paul public library, and we own about a dozen or so. They are well worth rereading.