The Pinhoe Egg by Diana Wynne Jones (The Chrestomanci Series: Book 7)

Spells always have consequences and it's Chrestomanci's job to make sure everything is safely under control. Even so, in the village around Chrestomanci Castle, all sorts of secret magical misuse is going on. And when Cat Chant finds the Pinhoe egg, chaos is just the beginning! A masterpiece of magic, mayhem and mirth!

9/10 A masterpiece of magic, mayhem and mirth!