House of Fire and Magic - Sherrilyn Kenyon

From #1 New York Times and international bestselling author Sherrilyn Kenyon comes an all new world unlike anything ever seen before...
TAKE BACK YOUR KINGDOM
For centuries, they've been bitter enemies and their kingdoms at war. But desperate times call for desperate measures. Tanis's beloved older brother and heir to their throne has been slain by a human who continues to flaunt her brother's death.
In order to restore her family's honor, she's willing to make a deal so that she can infiltrate the human world and recapture what was so cruelly taken.
But Dash Coeur de Noir is the most hated and feared of his breed and he's on his own mission when Tanis crosses his path- he must keep the kingdoms from destroying each other.
In a world built on magic, blood and betrayal no one can trust anyone. With a bounty on their heads and assassins and enemies out to claim them both, they must somehow learn to trust each other or watch their world burn.