Have you read Terry Pratchett?

The important thing is just to read them. You can always read them again in a different order. I would just suggest that you read the Light Fantastic and the Colour of Magic first because if to read them last I think they will feel less impressive.

But reading them in order of writing or publication has one specific benefit and that is that you follow pTerry on his journey as a developing writer. There is no doubt that later books such as The Night Watch and The Hogfather are more substantial works both of art and social commentary than the first two.