Forevermore (Darkest London #7) by Kristen Callihan

Miss Layla Starling, the young, beautiful, and extremely wealthy heiress, is the talk of London. Until now, she’s managed to evade the marriage noose. Despite the fact that she is unfortunately American, she’s received a staggering number of offers. And turned down every one. St. John Evernight does not want to admit the relief he feels every time he hears that she has rejected one of her suitors. Which is unfair of him. Layla deserves to be happy.

But he can not offer her happiness. He will never be normal, never be anything but a freak in her world. So St. John resolves to keep his distance, until he is recruited by the Society for the Suppression of Supernaturals to guard Layla. For she is in grave danger, and he is about to learn the full extent of his powers.