Marillier is a wonderful fantasy writer, and this tale a great addition to her brilliant Sevenwaters series. I believe (though I am not sure) that it is a retelling of the fairy tale Tam Lin. Clodagh, third daughter of Sean, Lord of Sevenwaters, has to go on a dangerous quest when her newborn baby brother is stolen by the Fair Folk and a changeling left in its place. Her courage, wisdom, and capacity for love will be severely tested as she faces the most powerful of the Fair Folk, the cold-hearted Mac Dara, Lord of the Oak, in the shadows of the Otherworld. As with its predecessors, this latest book of the series is filled with complex, fascinating characters steeped in the wonder of traditional fantasy. My only complaint is that her character list, which included the proper pronunciation for the names, was not complete; and I badly wanted it expanded to include places as well, because these Celtic names are an absolute bear to try to pronounce.
Great read with a great ending.