The Bargain of Thorns. Thorns series, #1

When a desperate mortal girl strikes a bargain with the fae, she expects danger-just not desire. Aria Wynn knows better than to trust the fae. But when her family's life hangs by a thread, she accepts a prince's impossible offer: serve one year in the twilight court of Evershade in exchange for their safety. Bound by ancient magic and a crown that feeds on truth, Prince Riven needs a mortal witness to keep his court from devouring itself-and Aria's defiance might be the only thing keeping him human.
In a kingdom ruled by oaths and haunted by hunger, every word is a weapon. As Aria learns to navigate the twisted beauty of fae politics, she finds herself drawn to the prince whose crown could kill him and whose touch burns brighter than the stars above his cursed realm. But bargains are never simple. When the courts of dawn and moon rise against Evershade, Aria must decide whether love is worth breaking the oldest law of all-that mortals never stay.