Apparently Pratchett is, too, but this news does worry me--for the unwritten books, yes, but also for the man and the life he's living. Still, you've got to love a man who can talk about an Alzheimer's diagnosis like this:
He told fans the statement should be interpreted as "I am not dead".
"I will, of course, be dead at some future point, as will everybody else," he said.
"For me, this may be further off than you think. It's too soon to tell.
"I know it's a very human thing to say 'is there anything I can do', but in this case I would only entertain offers from very high-end experts in brain chemistry."