The Princess and the Unicorn — Carol Hughes

Noble, with hooves and horn of gold, the unicorn is the embodiment of magic. When Princess Eleanor of England catches sight of him in Swinley Forest, she can't resist taking him back with her to Buckingham Palace. Unfortunately, once the unicorn leaves the forest, both he and the forest begin to sicken. As the only witness to the unicorn's departure, Joyce, an intrepid and curious young fairy, sets out for London on a grand adventure to rescue the unicorn--and maybe help the princess while she's at it.