^^ Seconded.
Theoretically you can read the novels in any order, but it's actually a lot more fun on the first go-round if you try to go through them in chronological sequence - you get to see the characters develop that way, and his characters are really worth watching unfold.
I will be forever grateful to the colleague who started me off with Guards! Guards! and Wyrd Sisters.
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"I believe you find life such a problem because you think there are the good people and the bad people," said the man. "You're wrong, of course. There are, always and only, the bad people, but some of them are on opposite sides."
-- Terry Pratchett, Guards! Guards!