I was wondering whether to suggest watching the TV adaptations?
I thought Going Postal was brilliant! but then maybe it helps to have read the book first and that's what makes the tv show so good?
From the books - i would recommend Guards! Guards! as a good starting point too. it's a great story by itself, full of fantastic characters. the rest of the books with those characters are great.
Then again, Wyrd Sisters might be a great place to start, as you'll be familiar with the underlying story that it is parodying.