Stranger by the Lake — Jennifer Wilde

Pretty Susan Marlow had always loved Greenwood Manor and its crusty, delightful mistress, her aunt, Lady Agatha Gordon. But now, when Susan returned to the stately old mansion, all seemed utterly changed. There was a man at Greenwood now, a tall, powerful, breathtakingly handsome man. His name was Craig Stanton, and Lady Agatha had made him her permanent guest. She called him a genius, a man who would shed new light on the Gordon family history, and the only one in the world she could trust.

But for Susan, Craig Stanton was something else...a man of strange powers who had woven a sinister spell over her aunt...a figure of mystery and dark and devious purposes...and a lover whom Susan was helpless to resist as he took her, trembling, in his arms...