The Wood Queen (The Iron Witch Trilogy, Book 2)

Donna Underwood is facing an alchemist tribunal for stealing the elixir of life, but when her ailing mother takes a turn for the worse and Donna seeks an audience with Aliette, the Wood Queen, she is offered a deal--if Donna can use her powers to open the door to Faerie and send the wood elves home, Aliette will free her mother from the elven curse that has shattered her mind.