Carpe Jugulam
In case you're despairing, thinking "I've read all the Discworld books except this one, and I can't find it anywhere!", don't worry - Carpe Jugulam is not available yet. It is due for release in November, when Terry Pratchett is undertaking another signing tour to promote it. More information about it can probably be found on alt.books.pratchett or the L-Space Web.