Discworld 14: Lords And Ladies

'People didn't seem to be able to remember what it was like with the elves around. Life was certainly more interesting then, but usually, because it was shorter. And it was more colourful if you liked the colour of blood . . .'
On Midsummer Night, dreams are especially powerful. So powerful, in fact, that they can cause the walls between realities to come crashing down. And some things you really don't want to break through.
The witches Granny Weatherwax, Nanny Ogg and Magrat Garlick return home to discover that elves have invaded Lancre. And even in a world of wizards, trolls, dwarfs, Morris dancers - and the odd orangutan - they're spectacularly nasty creatures.
The fairies are back - and this time they don't just want your teeth . . .

Lords and Ladies is the fourth book in the Witches series, but you can read the Discworld novels in any order.