Guards! Guards! is a good point to start at, as it's the first of the city watch "arc". Wyrd Sisters is the first of the witches. Wee Free Men is the first of the Tiffany Aching stories, I think there are four now? It's one of the most recently written, but stands alone enough from the rest of them. It was the first one I read. Here's a link to a (giant) nifty chart someone made. Nifty Chart If you haven't read The Name of the Wind yet, doooo iiitt.