Cuckoo in the Night — Pamela Kent

Janine Scott knew her sister was upset

Something was very wrong at Sandals, the isolated Devonshire house in which Janine's sister and brother-in-law lived.

And it was not comforting when a strange young man burst into her bedroom in the middle of Janine's very first night there.

Her sister had said her biggest fear was her recurring "nightmare man." But this man was real--and he seemed to think he had a perfect right to be there!