Rarity is the fourth pony to be featured in an original chapter book by G.M. Berrow, published by Little, Brown Books for Young Readers. The book isn't due out until April 8, 2014, but you can pre-order it now on Amazon for $5.39.
Description from the listing:
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!Other books in the series:
Twilight Sparkle and the Crystal Heart Spell
Pinkie Pie and the Rockin' Pony Palooza Party!
Rainbow Dash and the Daring Do Double Dare (January 7, 2014)