Making Money - Terry Pratchett

The Chief Cashier is almost certainly a vampire. There's something nameless in the cellar (and the cellar itself is pretty nameless), and it turns out that the Royal Mint runs at a loss. A three-hundred-year-old wizard is after his girlfriend, he's about to be exposed as a fraud, but the Assassins' Guild might get him first. In fact, a lot of people want him dead.