[Pernicious]

The first two Discworld books have always felt to me to be the most experimental of the series - Pratchett messing about before he has a clear idea of how to write the Discworld books. They are simple paradies of the most stereotypical Sword and Sorcery questlines - while his later books fleshes out a world and it's characters, with more emphasis on parodying modern life.

I often feel that Pratchett finds his feet in the fourth book, Mort, and the first three are books are almost non-canon. A lot of the places visited in the quest of the first two books are never mentioned again and the characters of Death and Granny Weatherwax (from book three, Equal Rites) are pretty much rewritten from scratch in Mort and Wyrd Sisters. From Mort onwards there is a consistency to his world, though Sourcery and Eric return to a story style similar to the first two.

Having said that, his early books are still very enjoyable and funny books - they are just a little bit jarring if you are used to reading his later works.