What confused me is that in The Fifth Elephant, mail coaches were running in Uberwald, and clacks towers were just going up, but in Going Postal the mail coaches JUST started running and went to Genua, and didn't run for 50 years previous, and all the clacks towers were up, and in the process of falling down again. Also, Vimes is specifically mentioned, and as Commander. That's why I was confused...
Anyway, I'm going to pick up the other Watch ones I missed, then the ones connected to Going Postal, and then I'll probably read the Death series.