The Pinhoe Egg

Cat Chant and Marianne Pinhoe have discovered something incredibly exciting, truly precious, and very strange--an egg.
This egg was not meant to be found. Chrestomanci himself, Cat's guardian and the strongest enchanter in the world, is sure to find it particularly interesting. And that's the "last" thing Marianne's family of secret rogue witches wants.
But the Pinhoes' secrets are falling to pieces, and powerful spells are wreaking havoc across the country-side. Marianne and Cat may be the only two who can set things right--if Marianne accepts her own powerful magic, and Cat solves the mystery behind the mystical Pinhoe Egg.