In the last few years, the author has been diagnosed with an early-onset form of Alzheimer's which started by attacking his cognitive centers dealing with vision. He's been able to continue to write by dictation, and using auditory cues.
Yesterday, I finished reading Snuff, which IIRC is his 39th novel in the Discworld series (so many that I can't keep track of the number...). I've found over these many books that all of them have something I've liked, and many were brilliant; this one is more the former than the latter. Near the end of the book, I found myself deeply touched, but at other points, things got a bit confusing. Still, that might have been intentional, as the scene described was one of great confusion and clatter.
I'd still recommend the book, though it's probably not one that stands alone well. As part of the saga, very good.