Firewalker (The Worldwalker Trilogy)

Lily Proctor, a witch, is back in her own universe, after she almost died in the Pyre that fueled her escape. She had enough presence of mind to transport herself and Rowan Fall from alternate Salem to her universe so she could heal. She knows that she must hide her magic for the safety of the world. Unfortunately, Lillian, ruthless ruler of the 13 Cities, is not willing to let Lily go that easily. If she can't persuade Lily to return to her world, she'll have to find a way to make her come back.