Guardian of Darkness by Lara Adrian

To repay a debt to an old friend, Breed vampire and former Hunter, Razor, has been drone surveilling a remote Colorado cabin for nearly a year, never imagining the beautiful woman he's become obsessed with from afar is anyone but his friend's lover, Laurel Townsend. When his mission turns urgent and Razor is tasked with retrieving Laurel from imminent danger, he arrives only to find the cabin destroyed and the woman inside brutally murdered. Vowing lethal justice against those responsible, Razor's hunt collides with free-spirited Willow Valcourt, Laurel's identical twin sister. The last person Willow should trust is a lethal predator like Razor, yet he is all that stands between her and the powerful, hidden enemies who killed her twin. A cross-country chase and cryptic clues left behind by Laurel send Willow and Razor on a journey that will take them from the twins' tragic past to the threshold of an international scheme to wage a secret, final war on the Breed. Drawn together by survival, haunted by loss, and surrounded by peril at every turn, Willow and Razor have only each other to prevent catastrophe. Yet as Willow surrenders to the needs only Razor can satisfy in her, and Razor vows to safeguard the true mate he never expected to find, their newfound passion will be put to the ultimate test. Contains mature themes.