Heart of the Moors by Holly Black

Ruling a kingdom is challenging under any circumstances. But Aurora -- queen of Perceforest and queen of the Moors -- is reigning under the weight of two crowns and is determined to sign a treaty to unify humans and the Fair Folk.
   But as spiky dark flowers grow fiercely around Perceforest's borders and a stable boy mysteriously disappears, tensions between humans and faeries are at a dangerous high.
   But who can she trust when it becomes apparent that an underhanded plot poses an unforeseen danger?
   It's up to Aurora to trust in her own heart and save her kingdom.