The fourteenth Discworld novel.
Reality is all very well in small doses. It´s a perfectly conventional and convenient way of neutralising the imagination. But sometimes when there´s more than one reality at play, imagination just won´t be neutralised, and the walls between realities come tumbling down. Unfortunately there´s usually a damned good reason for there being walls between them in the first place. To keep things out. Things who want to make mischief and play havoc with the natural order.