Seven Tears for Apollo — Phyllis A. Whitney

The Greek island of Rhodes, to which Dorcas Brandt flees in hope of escaping the strange incidents that have beset her since the death of her husband, Gino, in an airplane crash. She suspects that those who worked with Gino in his mysterious and illicit art dealings believe her to possess some knowledge which they must acquire at all costs. In Rhodes, with its ancient Greek ruins and medieval fortresses, Dorcas feels that she and her small daughter will surely find safety.
Her hopes are frustrated, for on the island singular happenings dog her steps. The strange woman who comes to care for the child watches Dorcas with hostile eyes. Marks of warning are scrawled on a balcony. The symbol of an owl surprises her in circumstances that terrify. The promise of new love is threatened. As imminent danger to the little girl becomes apparent, Dorcas's very sanity seems to hang in the balance.
On the acropolis of Lindos, where sheer cliffs plunge to the Aegean, the unknown comes into the open, and the forces of good and evil are meshed in final dramatic struggle.