The Riddle Of The Wren by Charles de Lint

A lyrical, character-driven fantasy about a young person drawn into a quest to solve an ancient, mysterious riddle that ties together past and present; as they travel through a landscape haunted by folklore and strange, lingering magic, they encounter allies and adversaries, confront tests of loyalty and identity, and must reckon with the costs of breaking old bargains in order to free themselves and others from a subtle, insidious power.