Lords and Ladies by Terry Pratchett (Discworld #14)

The fairies are back - but this time they don't just want your teeth …

It's Midsummer Night - no time for dreaming. Because sometimes, when there's more than one reality at play, too much dreaming can make the walls between them 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.

Granny Weatherwax and her tiny coven of witches are up against real elves. And they're spectacularly nasty creatures. Even in a world of dwarves, wizards, trolls, Morris dancers - and the odd orang-utan - this is going to cause trouble . . .

Book by author Terry Pratchett.