The journey home - Kathleen Duey

The long hard winter is at last over, but time is running out for Heart and her beloved unicorns.
Heart is convinced that the old man from Castle Avamir can help her find her family - the only people she dares to trust with the Unicorn's secret. But the old man has been taken prisoner by Lord Dunraven, and Heart's search for the truth is leading her into the most dangerous place of all: the depths of Dunraven's heavily guarded castle.
Heart's quest is nearing its end - and what awaits her is beyond her wildest dreams...