I have everything he wrote (which includes a number of non-Discworld books, like Johnny and the Bomb, and The Bromeliad Trilogy).
I didn't especially like the Science of Discworld books that much, but he didn't really write them.
One character that showed up in every one of his Discworld books -to a point- was Death.
After Sir Terry got his diagnosis, I noticed that Death stopped showing up in the books.
These were the last posts from Sir Terry's Twitter account, March 12th 2015:
AT LAST, SIR TERRY, WE MUST WALK TOGETHER.
Terry took Death’s arm and followed him through the doors and on to the black desert under the endless night.
The end.