I am really sad to see Terry Pratchett has gone a big miss for the DL. I wonder why he was taken off the list given that he had to withdraw from public life last July. Alzheimer's is extremely aggressive, from personal experience my father went from being able to speak and hold conversations to not being able to do much more than repeat what we said to him in the space of 12 weeks and stopped speaking entirely 6 months later. It's an extremely nasty disease. 5 months before my dad died he was unable to feed himself, recognise his surroundings or communicate at all, it was so fast, he was bedridden and in the end started to aspirate when being fed as he had forgotten how to swallow. It was a lower respiratory infection that took my wee dad and I see the official cause of death for Mr. Pratchett is 'chest infection', much the same thing.
In response to questions about where to start with Terry Pratchett's books I don't know, how long is a piece of string. I can give you my experience and it's up to you where you want to start. Back in 1987 I was off work with full on dose of flu, still living at home and I was bored and miserable and making my parents and my siblings lives misery. One of my brothers, chucked The Colour of Magic at me and said 'you should read this it might make make you smile and give us all a break'. I retreated to my bed and read it and just could not put it down, I loved it thought it was great. I passed it onto my dad who thought it was brilliant and then to my mum who just loved it. My brother was happy to pass on the sequel to us 'The Light Fantastic' and we all eagerly awaited the release of Equal Rites in paperback. I was almost first in the bookstore in Glasgow - no internet then it was John Smith's in St. Vincent Street and greedily took it home when it was first released. I was disappointed, I have never reread it and perhaps I should. I have had over the years had a hit and a miss love affair with Pratchett's Discworld. I loved Guards! Guards! and hated Soul Music. I adored Mort and enjoyed Wyrd Systers and Moving Pictures. I grew tired of a new book every 6 months and stopped getting them for birthday and Christmas presents well over 10 years ago. They lay in my bookshelves unread. When I moved most made their way to a local charity shop but I have kept my brother's original copies of The Colour of Magic and The Light Fantastic that he lent me 28 years ago, they are squirreled away in my collection. My mum recently picked up a copy of 'The colour of magic from a charity shop and enjoyed it just as much as she did in 1987. Not sure if this helps with choices of where to start.