Shadowsong (Book 2 of the Wintersong Duology)

Six months after the events of Wintersong, Liesl is back in the world above, trying to advance her own and her brother Josef’s musical careers. However, her brother has become cold and distant, and Liesl cannot forget the Goblin King she left behind beneath the earth. When ominous signs reveal that the barrier between worlds is crumbling, she must return to the Underground to solve the mystery of life, death, and the true identity of the Goblin King.