Linda Chapman 
My Secret Unicorn: The Magic Spell 

Imagine sitting on the back of a snow-white unicorn. Imagine riding it high into the sky, the wind in your hair. Imagine owning it, loving and cherishing it forever. This dream comes true for Lauren Foster. By day she rides Twilight, an ordinary little pony, but as night draws in and Lauren recites a secret spell, Twilight becomes a unicorn with magical powers! Only Lauren knows Twilight’s secret.