The Twice Lost (The Lost Voices Trilogy #3) http://www.yabookscentral.com/media/reviews/photos/thumbnail/200x285s/27/ec/3c/_twice-1362953566.jpg
There she finds hundreds of mermaids living in exile under the docks of the bay. These are the Twice Lost: once-human girls lost first when a trauma turned them into mermaids, and lost a second time when they broke mermaid law and were rejected by their tribes. Luce is stunned when they elect her as their leader. But she won’t be their queen. She’ll be their general. And they will become the Twice Lost Army—because this is war.