This was a rumoured Discworld novel, anticipated as a third Moist von Lipwig book, and first expected for release after I Shall Wear Midnight. It would supposedly have involved Moist being tasked by the Patrician to rework Ankh-Morpork's system of taxation. However, this was not to be; the eventual third book to feature Moist - and the penultimate book of the entire Discworld series - was announced live via Skype to Nullus Anxietas IV, the Australian Discworld Convention, on 8 March, 2013 to be Raising Steam. According to the official @terryandrob Twitter account, the livestream was interrupted after Terry held up the first word, leading to them clarifying: "But it's definitely not Raising Taxes."
Given comments made by Terry at an appearance at the Hay-on-Wye Literary Festival in 2011 or 2012, this was not the only eventually unwritten Moist book he was considering.