Moonkind by Sarah Prineas

Fer must save her realm from the Forsworn, a group destroying the land with their broken oaths, in Moonkind, the conclusion to award-winning author Sarah Prineas's fantasy-adventure series, which includes Winterling, Summerkin, and the short story Thrice Sworn.

As the Lady of the Summerlands, Fer has vowed to serve her people without the deception of the glamorie, and she had trusted other leaders to fulfill the same promise. But not all of the Lords and Ladies want to keep their oaths, and they've unleashed the consequences of their betrayal onto the lands. Only Fer, with the help of the puck-boy Rook, can fight the stillness invading the realm. But can she trust Rook? And can she protect her people before it's too late?

Sarah Prineas combines a brave and resourceful young heroine with a richly detailed fantasy world and beloved folklore into a story that will delight middle-grade fans of Diana Wynne Jones, Ingrid Law, and Rick Riordan.