Heir to Sevenwaters by Juliet Marillier
The chieftains of Sevenwaters have long been custodians of a vast and mysterious forest, one of the last refuges of the Tuatha De Danann, the Fair Folk of ancient story. Human and Otherworld dwellers have existed there side by side, separated by a thin veil between worlds and sharing a wary trust. Until the spring when Lady Aisling of Sevenwaters finds herself expecting another child, and everything changes.
With her mother pregnant, Clodagh fears the worst as Aisling is well past the safe age for childbearing. Her father, Lord Sean of Sevenwaters, faces his own difficulties as warring factions threaten his borders. When Aisling gives birth to a son-a new heir to Sevenwaters-the responsibility of caring for the infant falls to Clodagh while her mother recovers.
Then the family's joy turns to despair when the baby is taken from his room and something . . . unnatural is left in his place. To reclaim her brother, Clodagh must enter the shadowy Otherworld and confront the powerful prince who rules there. Accompanied on her quest by a warrior who may prove to be more than he seems, Clodagh will have her courage tested to breaking point. The reward may be far greater than she ever dreamed . . .