Between The Blade And The Heart By Amanda Hocking

As one of Odin's Valkyries, Malin's greatest responsibility is to slay immortals and return them to the underworld. The balance of the world rests on her ability to carry out orders. But when Malin discovers that her mother spared the life of an immortal who was destined to die, her world is thrown into chaos.
As Malin wrestles with the knowledge that her mother might not be who she thought, she’s also thrust into the path of a gorgeous blue-eyed guy named Asher who needs her help slaying the rogue immortal who destroyed his family. Malin, along with her best friend and her ex-girlfriend, must decide where her loyalties lie...and whether helping Asher enact his revenge is worth the risk—to the world and to her heart.