A Hat Full of Sky

The second book in the Tiffany Aching series - five Discworld novels by master-storyteller Terry Pratchett.
Tiffany Aching is going 'into service': to be a lady, no less, a maid in a big house. At least, this is what she tells her parents.
Really, Tiffany is going away to learn magic.
But making friends with fellow witches is always difficult when an invisible-being-that-cannot-be-killed takes over your body - stealing money, and threatening violence.
Tiffany must use all her witchy cunning to reclaim what's hers. Luckily, she has a bit of help. What's tiny, Scottish and blue all over? A Nac Mac Feegle of course - the rudest type of fairy, and handy to have in a tight spot . . .