I will miss waiting on this next instalment of his books. However, he appears to have completed a Discworld novel in 2014....
He completed his last book, a new Discworld novel, in the summer of 2014, before succumbing to the final stages of the disease. http://www.pjsmprints.com/
So it looks like we have one final book to come from him, just think, a Discoworld novel that will not be signed by the great man.