Love Disk World. I didn't know about the diagnosis. 
He plans on continuing as long as he can. I hope that is a good long time, since I for one would really miss his writing!
Pratchett is probably my Favorite author right now, absolute tragedy about his Early Onset Alzheimers.
He is one of my top favorites! Though my normal fare is more the Baen publishing stable of Authors, David Drake, David Weber, John Ringo, Travis Taylor, Elizabeth Moon, (who is actually 3 days older than me), They have a whole bunch of good ones and most of their stuff is available at webscriptions in non-DRM ebook formats! I never miss a Grabtville Gazette!
I can't believe they didn't include the Rift War novels by Raymond E. Feist. If you like fantasy, I highly suggest you pick up these books. His Empire series, covering the Rift War from the other side, is also quite amazing!
So many to choose from .. some of my favorites are
Frank Herbert Dune series
Conan Series - by Robert Howard - these are more short stories Pulp fiction style
The Oz Books - Frank Baum are great
The Chronicles of Narnia
Dune wouldn't be on this list as it is Science Fiction rather than Fantasy! Though I agree it should be on a top ten somewhere! Have you read "Guns of the South" by Harry Turtledove? Think that was his high point!
Haven't read Guns of the South .. looking at Amazon it looks pretty interesting.but have read
If The South Had Won The Civil War and one of his Atlantis books, I think the first one.
also have read The Disunited States of America.
I have a couple more EBooks by him that I have not read yet - Curious Notions, Gladiator and High Places
I've read a lot of his stuff, even the ones he co-wrote with Gingrich! 
I think Guns of the South was the best of the bunch! He knows his History and is a decent writer, but this one stands above the rest as that rare "page turner" that you can't put down!
lamparty
Not a real big fan of Fantasy, but terry Pratchett's Disk World series is really good reading on the light side! Too Bad he has been diagnosed with early stages of Alzheimer's!!