My Secret Unicorn: The Magic Spell and Dreams Come True by Linda Chapman
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 and before long they discover that his special magic can help other people overcome their problems.
The Magic Spell and Dreams Come True brings together the first two adventures in the My Secret Unicorn series in which Lauren discovers Twilight's secret and, using a little magic, helps a friend in danger.