Rarity and the Curious Case of Charity (Used Book) - G. M. Berrow
Rarity is excited to welcome her new apprentice, Charity, to Ponyville. Rarity is sure that the two of them will become best friends, bonding over their love of fashion and glamour. But after Charity dyes her mane and tail to look more like her mentor, and begins to copy the fashionista's every move, Rarity realizes this pony is a problem!
My Little Pony: Friendship Is Magic, #4