I've just bought "Where's My Cow?" For some reason it was missing from my collection but not any more.
I tend to re-read in chronological order if I'm going for the whole lot - but sometimes I'll pick out the groups, usually the Witches. Or the Watch, of course - but they are more intertwined with the others, I find.
Fmarf - you might not have known him but you can still feel the loss of someone who enriched your life - tell your DD that, if you haven't found other words yet. 
There was another author who was reading Terry Pratchett when he died - Robert Asprin. He was only 61, died in bed of a heart attack and his reading glasses and a TP book were found next to him.
Oh I'm still weeping. :(