This is very sad news indeed. Aside from being very funny, his books have a lot of heart in them. (As well as a very evil cat, Greebo, whose response to everything is to eat it, fight it, or mate with it--sometimes more than one option.) He can go from laugh-out-loud funny to serious very smoothly, without either part jarring.
He's known for being very approachable with his fans and just seems like a great guy all around.
The first book of his that I read was Wyrd Sisters, a wonderful take on Macbeth. It's one of his lightest. If you want something with more of an edge to it, Small Gods; Guards, Guards; or Lords and Ladies are all good.