Legion by Julie Kagawa

Dragon hatchling Ember Hill was never prepared to find love at all—dragons do not suffer human emotions—let alone the love of a human and a former dragonslayer, at that. With ex-soldier Garret dying at her feet after sacrificing his freedom and his life to expose the deepest of betrayals, Ember knows only that nothing she was taught by dragon organization Talon is true. About humans, about rogue dragons, about herself and what she’s capable of doing and feeling.

In the face of great loss, Ember vows to stand with rogue dragon Riley against the dragon-slaying Order of St. George and her own twin brother Dante–the heir apparent to all of Talon, and the boy who will soon unleash the greatest threat and terror dragonkind has ever known. Talon is poised to take over the world, and the abominations they have created will soon take to the skies, darkening the world with the promise of blood and death to those who refuse to yield.