Lord of Sin — Susan Krinard

Nuala is descended from ancient witch folk, eternally bound to help others find love. But after the death of her husband, she harbors no such dreams for herself. Then she meets Sinjin, the Earl of Donnington, and feels something stir within her for the first time in centuries....

Handsome and scandalously tempting, Sinjin has never met a woman he couldn't seduce. Yet from the moment he sees the stunning young widow, he knows he wants more than just one night of sin -- and even the discovery of the dark secret they share won't stop him from trying to possess her forever. But first he must free her from her immortal bondage, which means robbing her of her magic for all time....