Iron Legends by Julie Kagawa
Never make a promise to a faery. They always come to collect. Now Meghan Chase must fulfill her promise to Prince Ash of the Winter Court and embark upon a dangerous journey into the heart of enemy territory—while being pursued by a relentless new foe and guarding her own foolish heart.
What can turn enemies into reluctant allies? A call from the Exile Queen, Leanansidhe, ties legendary prankster Puck to his archenemy, Prince Ash, on a journey that may end in betrayal and will set them both on an irreversible path.
Before she ever knew what she might become, Iron Queen Meghan Chase was warned by the oracle that her firstborn child would bring nothing but grief. And even as Meghan and Ash celebrate their long-awaited union, the prophecy stirs.