From goodreads.com:
Long ago, when immortals walked the green hills of Ireland, Caer's father, King Ethal, cast a terrible spell on her. Lest she marry a mortal man, Caer was condemned to live every second year in the form of a swan. At first she didn't mind drifting on the cold, empty waters of Loch Bel Dracon, but one day a young man came to the lake's shore, and she fell in love with him. Could she win the heart of Aengus? Or was Caer destined to be lonely forever? Ted Nasmith's detailed illustrations, based on ancient Celtic signs and symbols, enrich Joanne Findon's haunting retelling of a medieval Irish tale.