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.