Tanith Lee 
Heart-Beast 

Where the moon washed the deck, something sat, its huge head raised, more like the visage of a bear than a wolf, yet long-snouted, the jaws open. Cold eyes that had no soul in them mirrored the moon…After the killing of his brutal father, golden and handsome Daniel Vehmund has sought refuge in an exotic faraway land. But his contentment is shattered when a fabulous diamond is pressed on him by a sinister graverobber. From that first fatal contact, Daniel is doomed each night of the fill moon to become something else…something dark and powerful and savage. And nothing that originates on this earth can destroy him.Laura, a beautiful redhaired farm-girl, lives with her two awful sisters and her selfish grasping parents in the remote English countryside. Apparently condemned to a life of drudgery, all is transformed when she is wooed and wed by a wealthy local squire. But this gilded existence is threatened when a travelling magician persuades her besotted husband to buy from her a very unusual diamond. For this gem is "The Wolf", the diamond that had transformed Daniel Vehmund’s destiny. And when he himself returns at last to England, his fate and Laura’s are devastatingly intertwined…Meanwhile some dark, malignant presence continues to prowl the woodlands and hedgerows…and seemingly nothing can curb its unquenchable bloodlust.