Look, we already know he's writing another Discworld book, and that it's *not* Raising Taxes. Tweets from a month or so ago confirm this. He hasn't said what it is yet. Possibly it'll be an end-of-2013 release, but I'm suspecting perhaps it'll be out in 2014 instead.
Nullus Anxietas V - The Australian Discworld Convention
10-12 April 2015
Novotel Parramatta, New South Wales, Australiahttp://ausdwcon.org