Diminished (The Elder Tree Trilogy #2) by Erica Eberhart:
 
A queen haunted by ghosts. A dragon cursed in human skin. A kingdom on the brink of war.

In the aftermath of her father's death, Queen Greer stands at the edge of grief and revelation. Magic—once forbidden—has returned to Wimleigh, but so have enemies. When a prophetic funeral warns of rising death, it sets into motion a chain of events that will test the mettle of queens, heirs, and warriors alike.

As Greer grapples with panic attacks and the vengeful ghost of her father, her sister, Caitriona, struggles under the weight of monstrous expectations. Bearing horns, scales, and the voice of a dragon in her mind, Caitriona must lead a secret mission with Ailith MacCree—now a royal guard—north to find a lost prince who could stop a war—if he’s willing to fight.

But treachery weaves through the forests, poisons creep in, and fae assassins lurk in the shadows. With kingdoms falling to darkness, an ancient tree bleeding out its final life, and bonds shattered by betrayal, Wimleigh’s hope rests on a dragon no longer afraid to rise.

When magic is reborn and monsters wear crowns, who dares to lead the charge?