Every Discworld book from 1 to 11 (need ...more!)
and yes, artermis foul is brilliant, eoin is a great author. get the wish list, because that's even better, and try the supernaturalists.
back to discworld.
discworld inspired me to write my own books, he is one of the funniest people writing today. Pyramids was excellent, as was Moving Pictures, Colour of Magic, while simple is still one of his best, and Light Fantastic is another one of my favourites.
Also, the Wyrd Sisters is a scream too, making Macbeth into a golariously up to date story.